Zoological Philosophy
J. B. Lamarck

[This translation, which has been prepared by Ian Johnston of Malaspina University-College, Nanaimo, BC, Canada, is in the public domain  and thus may be used by anyone, in whole or in part, without charge and without permission, provided the source is acknowledged, released September 1999]

Table of Contents


First Part

Considerations of the Natural History of Animals, Their Characteristics, Their Interrelationships, Their Organic Structure, Their Distribution, Their Classification and Their Species

Chapter Six

Degradation and Simplification in the Organic Structure from One Extreme to the Other of the Chain of Animal Life, Going from the Most Complex to the Simplest

Among the considerations of interest to Zoological Philosophy, one of the most important concerns the degradation and the simplification observed in the organic structures of animals, as we move from one extremity of the chain of animal life to the other, from the most perfect animals to those with the simplest organic structure.

Now, the question is whether this fact can be truly verified. For then it will strongly illuminate for us the plan nature has followed and will set us on the road to discovering several of the natural laws most important for us to understand.

I propose here to prove that the fact in question is reliable and the product of a constant natural law, which always works uniformly, but that a particular cause, easily recognizable, make the regularity of the results which this law ought to produce vary here and there throughout the entire extent of the chain of animal life.

To begin with, one is forced to recognize that the general series of animals, arranged in accordance with their natural affinities, displays a series of particular groups, resulting from the different systems of organic structure employed by nature, and that if these groups are themselves organized in accordance with the decreasing complexity of organic structures, they form a real chain.

Then, it is noticeable that, in spite of the anomalies (whose cause we shall establish), from one extremity to the other of this chain there reigns a striking degradation in the organic structures of the animals which comprise it, and a proportional diminution in the number of faculties of these animals. The result is that, if at one end of the chain in question are the most perfect animals in all respects, then at the opposite end we necessarily see the simplest and most imperfect animals which can be found in nature.

Finally, one has reason to believe, through this analysis, that all the special organs are simplified progressively from class to class, are changed, become diminished and weaker gradually, and that they lose the place where they are concentrated, if they are of primary importance, and end up by being completely and utterly done away with, before having reached the opposite end of the chain.

True, the degradation I am talking about is not always a regularly modulated progression. For often some organ is missing or changes suddenly, and in its transformations, it sometimes assumes odd shapes which do not link up with any other to any recognizable degree. Again, often some organ disappears and reappears several times before being utterly done away with. But we are going to see that this could not have been otherwise, that the cause which creates organic structures progressively must have experienced various deviations in its products, because its products are often in the position of being changed by a foreign cause which works on them with an efficacious power. Nonetheless, we will see that the degradation under discussion is no less real and progressive in all the examples wherever we have been able to see it.

If the cause which constantly tends to increase organic complexity was the only one which had influence on the animals' shape and organs, the increasingly complexity in organic structure would be very regular throughout in its progression. But things are not like this. Nature found herself forced to submit her work to the influence of circumstances which operate on it, and everywhere these circumstances made the products vary. That is the particular cause which brings about here and there in the course of degeneration which we are going to confirm the often bizarre deviations which it presents to us.

Let us attempt to clarify both the progressive degeneration in the organic structure of animals and the cause of the anomalies which the progress of this degeneration manifests along the series of animals.

It is evident that, if nature had brought to life only aquatic animals and if all these animals had always lived in same climate, the same sort of water, at the same depth, and so on, then without doubt one would have found in the organic structure of these animals a regular gradation, even nicely modulated.

But nature has not confined her power within such limits.

First, one must note that nature has varied considerably the conditions of the water itself: fresh, salt, tranquil or stagnant, running or constantly agitated, hot and cold, finally, shallow and very deep, thus presenting special circumstances each of which has a different effect on the aq1uatic animals living there. Now, to a degree corresponding to their organic structure, the animal races found exposed to each of these conditions have undergone from them specific influences and thus have been diversified.

After nature had produced aquatic animals of all ranks and varied them remarkably with the help of the different circumstances given by the waters, those which she led gradually to live in the air, at first on the waters' edge and afterwards on all the dry parts of the earth were placed over time in circumstances very different from the first, conditions which so strongly influenced their habits and organs that the regular gradation which they ought to present in the complexity of their organic structure was remarkably changed, so that it is almost unrecognizable in plenty of locations.

These considerations which I have examined for a long time and which I will set out with reliable proofs, led me to present the following zoological principle, whose foundation seems to me to be safe from any challenge:

The progress in the complexity of organic structure in the general series of animals undergoes, here and there, anomalies brought about by the influence of environmental factors and of acquired habits.

In considering these anomalies, people have justified their rejection of the progression which clearly exists in the complexity of animals' organic structure and their refusal to recognize the progress which nature follows in her production of living bodies.

However, in spite of the apparent gaps (which I am going to point out), the nature's general plan and the uniform progress in her manner of working, although infinitely various in its means, are still very easy to make out. To succeed in that, one needs to look at the general series of known animals, first envisaging it in its totality and then in its large groups. One will see there the least doubtful proofs of the gradation which nature has followed in the design of organic structure, a gradation which the anomalies I have mentioned would never permit one to mistake. Finally one will observe that wherever extreme changes in the circumstances have not been at work, one finds this gradation perfectly modulated in the various sections of the general series to which we have given the name families. This truth becomes even more striking in the study of what we call species. For the more we observe, the more our specific distinctions become problematic, complicated, and minute.

Thus, the gradation in the design of animal organic structure will be a fact which we will not be able to cast doubts upon, since we will have provided detailed and reliable proofs of what has just been outlined. Now, since we are taking the general series of animals in the inverse order to the one nature herself followed in bringing them successively into existence, this gradation is then changed, for us, into a remarkable degradation which governs from one extreme to the other of the animal chain, except for the interruptions which result from objects we have yet to discover and those provided by anomalies produced by extreme environmental circumstances.

Now to establish by reliable facts the basis for the degradation in the organic structure of animals from one extremity of their general series to the other, let us first glance at the total make up of this series. Let us consider the facts laid out before us, and later we will move on quickly to review the fourteen classes which are its main divisions.

In examining the general distribution of animals in the way that I have presented it in the previous section, whose totality is unanimously vouched for by zoologists, who only argue about the limits of certain classes, I call attention to a very evident fact which should be, by itself, already decisive for my purpose, as follows:

At one end of the series (the one which people conventionally consider the anterior extremity), we see animals most perfect in all respects, whose organic structure is the most complex; whereas, at the opposite end of the same series are found the most imperfect animals in nature, those whose organic structure is the simplest and which one suspects are hardly endowed with animal life.

This well acknowledged fact, effectively beyond argument, becomes the first proof of the degradation which I intend to establish. For it is the essential condition for that degradation.

Another fact which arises from a consideration of the general animal series and which provides a second proof of the degradation which governs in the organic structure from one extremity of that chain to the other is the following:

The first four classes of the animal kingdom display animals generally provided with a vertebral column, while the animals of all the other classes totally lack this feature.

We know that the vertebral column is the essential basis of the skeleton, which cannot exist without it, and that wherever a vertebral column is found, there is a more or less complete skeleton, improved to a greater or lesser extent.

We also know that the improvement of the faculties proves the improvement in the organs which give rise to them.

Now, although man is beyond ranking, because of the extreme superiority of his intelligence, so far as his organic structure is concerned, he surely exemplifies the greatest improvement which nature has been able to reach. Thus, the more an animal's organic structure approaches man's the more it is improved.

This being the case, I observe that the body of a human being possesses not only an articulated skeleton but also one which is everywhere the most complete and improved in all its parts. This skeleton strengthens man's body, provides numerous points of attachment for his muscles, and allows him to vary his movements almost infinitely.

With the skeleton appearing as the main part in the design of the organic structure in the human body, it is evident that all animals furnished with a skeleton have an organic structure more perfect that those who lack such a structure.

That is the reason why the invertebrates are more imperfect than the vertebrates and why, when we place the most perfect animals at the head of the animal kingdom, the general animal series displays a real degradation in organic structure, for after the first four classes, all the animals in those which follow lack a skeleton and are, consequently, less perfectly structured organically.

But that is not all. Among the vertebrates themselves, the degradation in question is noticed again; later we shall see that we come across it among the invertebrates. Thus this degradation is a consequence of the constant plan which nature follows and at the same time a result of the order we are following (but inversely). For if we were to follow the order itself, that is to say, if we moved through the general series of animals going up it from the most imperfect right to the most perfect of them, rather than a degradation in the organic structure, we would find a growing complexity, and we would see successively the animal faculties increasing in number and improvements. Now, to prove throughout the degradation in question, let us at this point quickly move through the different classes of the animal kingdom.


Animals with mammary glands, having four articulated limbs and all the essential organs of the most perfect animals; with hair on some parts of the body

The mammals (mammalia, Lin[aeus]) must obviously be located at one of the extremities of the animal chain, at the one which displays the most perfect animals, the most rich in organic structure and faculties. For it is uniquely among them that we find those having the most fully developed intelligence.

If the improvement in the faculties establishes the improvement of the organs which give rise to them, as I have already said, in that case all the mammals, who are truly viviparous, therefore have the most improved organic structure, because it has been recognized that these animals have more intelligence, more faculties, and a more perfect combination of senses than all the others. In addition, there are those of them whose organic structure comes closest to that of human beings.

Their organic structure displays a body strengthened in its parts by an articulated skeleton, more generally a complete one in these animals than in the vertebrates of the three other classes. Most mammals have four articulated limbs, extensions of the skeleton. And all have a diaphragm between the chest and the abdomen, a heart with two ventricles and two auricles, warm red blood, free standing lungs contained in the chest, into which all the blood passes before being sent to other parts of the body. Finally, these are the only viviparous animals, for they are the only one in which the fetus, enclosed in its layers, nevertheless always communicates with its mother and develops by relying on her materials, and the young, after their birth, are nourished, for some time yet, from the milk of her breasts.

Thus, the mammals must occupy the first rank in the animal kingdom, from the point of view of the improvement in their organic structures and the large number of faculties (Recherches sur les Corps vivans, p. 15), because after them we do not find any more true viviparous reproduction, lungs contained by a diaphragm in the chest taking in all the blood which must be sent to other parts of the body, and so on.

In truth, among the mammals themselves, it is quite difficult to differentiate what belong essentially to the degradation we are examining from what is produced by environmental circumstances, ways of life, and long established habits.

However, we find even among them traces of the general degradation in organic structure, for those whose limbs are appropriate for seizing objects are more highly perfected than those whose limbs are only appropriate for movement. In fact, it is among the first that human beings, considered from the perspective of organic structure, are located. Now, it is evident that the organic structure of human beings is the most perfect and must be looked upon as the standard against which we must judge the improvement or the degradation of other organic structures in animals.

Thus, in the mammals, the three sections which divide this class (although unequally) display amongst them, as we are going to see, a perceptible degradation in the organic structure of the animals making them up.

First Section: Unguiculate Mammals have four limbs, claws which are flattened or pointed at the end of their digits but which do not surround the digits. These limbs are, in general, appropriate for seizing objects or at least hanging from them. Among these animals are found the animals with the most improved organic development.

Second Section: Ungulate Mammals have four limbs, and their digits are entirely covered at the extremities with a round horn called a hoof. Their feet are used only for walking or running along the ground, and are not capable of being used for clambering up trees or seizing an object or prey, or to attack and rip other animals. They feed themselves only on plant life.

Third Section: Exungluate Mammals have only two limbs, and these limbs are very short, flattened, and shaped for swimming. Their digits, surrounded with skin, have neither claws nor hooves. Of all the mammals, these are the ones whose organic structure is the least perfect. They do not have a pelvis nor rear feet; they swallow without preliminary chewing; finally, they habitually live in water, but they come to the surface to breathe air. They have been given the name cetaceans.

Although amphibians also live in water, from which they emerge from time to time to crawl around the shoreline, they actually belong to the first section of the natural order and not to the one which includes the cetaceans.

From now on, we see that it is necessary to differentiate between the degradation which comes about from the influence of the habitual environment and acquired habits and that which is the result of a less advanced progress in improvement or in the complexity of organic structure. Also, one has to be careful about descending into consideration of detail, because, as I will show, the environments in which animals typically live, the specific locations they inhabit, patterns of behaviour enforced by circumstances, ways of life, and so on, have a large power of modifying the organs. Thus, one could attribute to the degradation we are discussing shapes of parts which are really due to other causes.

It is evident, for example, that the amphibians and the cetaceans, living habitually in a dense medium, where well developed limbs would only be able to interfere with their movements, must have only very shortened limbs, that it is only as a result of the influence of the water impeding movements of very long limbs with solid interior parts, that they necessarily are made the way they really are, and that consequently these animals owe their general form to influences of the environment in which they live. But with respect to the degradation which we are seeking to recognize in the mammals themselves, the amphibians must be distant from the cetaceans, because their organic structure is much less debased in its essential parts. It requires that we bring them close to the order of unguiculate mammals, whereas, the cetaceans must form the last order of the class, since they are the least perfect mammals.

We are going to move on to the birds. But before that, I should note that between the mammals and birds there is no merging. There exists a gap to fill. Undoubtedly, nature has produced animals which almost fill up this gap and which must form a specific class, if they could be known, either in the mammals or in the birds, according to their systemic organic structure.

This has just been acknowledged by the recent discovery of two types of animals from New Holland, as follows:



Monotremes, Geoff[roy].

These animals are quadrupeds, without mammary glands, embedded teeth, or lips; they have only one orifice for the genital organs, excrement, and urine (a cloaca). Their bodies are covered with hair or bristles.

These are not mammals at all, for they do not have mammary glands and are very obviously oviparous.

They are not birds, for their lungs are not pierced and they do not have limbs shaped into wings.

Finally, they are not reptiles, for their heart with two ventricles necessarily distances them from the reptiles.

Thus, they belong to a special class.


Animals without mammary glands, with two feet and two arms shaped into wings; feathers cover the body.

The second rank obviously belongs to the birds, for if we do not find in these animals such a large number of faculties and as great an intelligence as in the animals of the first rank, they are the only ones except for the monotremes, who have, like the mammals, a heart with two ventricles and two auricles, warm blood, a skull cavity entirely filled by a brain, and the trunk always contained in ribs. Thus, they have qualities exclusively in common with mammalian animals and, consequently, affinities which we cannot find again in any of the animals of the later classes.

But the birds, in comparison with the mammals, display in their organic structure a clear degradation, one which has nothing to do with the influence of any sort of circumstances. In effect, they basically lack mammary glands, organs possessed only by the animals of the first rank, which serve for a reproductive system not found in the birds or in any of the animals in the ranks which come after. In a word, they are essentially oviparous, for the system of truly viviparous reproduction, something unique to the animals of the first rank, does not recur from the second rank on and does not reappear elsewhere. Their fetus, enclosed in an inorganic envelope (the shell of the egg), hardly communicates any more with the mother and can develop within the eggshell without feeding on her material.

The diaphragm which in the mammals completely separates the chest from the abdomen (although more or less obliquely) ceases to exist in this group or is found only in very incomplete form.

There is nothing mobile in the vertebral column of birds, except the neck and tail vertebrae, because the movements of the other vertebrae in this column, not being found necessary for the animal, were not carried out and did not hinder the significant development of the sternum, which now makes such movements almost impossible.

In fact, the sternum of birds, which forms a place of attachment for the pectoral muscles which powerful movements, carried out almost continually, have made very thick and strong, has become extremely large and carinate in the centre. But this concerns the habits of these animals and not the general degradation which we are looking at. This last point is valid because the mammal which we call the bat also has a carinate sternum.

All the blood of birds passes once more through their lung before reaching the other parts of the body. Thus, they breathe completely by their lung, like the animals of the first rank. After them, no known animal has a similar system.

But here we see a very remarkable peculiarity relevant to the circumstances where these animals are found. Living more than the other vertebrates in the air, in which they rise up almost all the time and cross in all kinds of directions, the habit they have acquired of puffing up air into their lungs in order to increase their volume and to make themselves lighter has made this organ stick to the side parts of the chest cavity and has put the air held there and rarefied by the heat of the location in a position to pierce the lung and the surrounding layers and to penetrate into all parts of the body, into the interior of the large bones, which are hollow, and right into the tubes of the large feathers (1).  Nevertheless, it is only in the lung that the blood of the birds receives the influence of the air which it needs; for the air which penetrates into the other parts of the body has another purpose than respiration.

Hence, the birds, reasonably placed after the mammalian animals, display in their general organic structure a clear degradation, not because their lung presents a peculiarity which we do not find in the first animals and which is due, like their feathers, only to the habits they have acquired of ascending into the air, but because they no longer have the system of reproduction appropriate to the most perfect animals and have only the system of the majority of animals in the later classes.

It is very difficult to recognize among the birds themselves a degradation in the organic structure which is the purpose of our research here. Our knowledge of their organic structure is still too general. Thus, up to now, one or other of the orders of this class has been arbitrarily placed at the head of it, and the class terminated in the same way with the order which someone wished to select.

However, if we consider that aquatic birds (like those with webbed feet), the wading birds, and the gallinaceans have an advantage over all other birds, in that their young, in coming out of the egg, can walk and feed themselves, and, above all, if one attends to the fact that, among the webbed-footed birds, the penguins and king penguins, in which the almost featherless wings are only oars for swimming and are not capable of flight, a feature which makes these birds similar, in a way, to the monotremes and the cetaceans, then we will recognize that the palmipeds, wading birds, and gallinaceans must constitute the three first orders of birds and that the doves, passerines, birds of prey, rapaces and climbers, must form the four last orders of the class. Now, what we know about the habits of the birds of these last four orders tells us that their newborns, emerging from the egg, cannot walk or feed themselves on their own.

Finally, if following this analysis, the climbers make up the last order of birds, since they are the only ones which have two digits at the back and two in front. This characteristic, common to them and the chameleon seems to justify our placing them close to the reptiles.


Animals with only one ventricle in the heart and still possessing a respiratory lung, but an incomplete one; their skin is smooth or furnished with scales.

In the third rank are placed, naturally and necessarily, the reptiles. They are going to provide us with new and greater proofs of the degradation in organic structure from one end to the other in the animal chain, starting at the most perfect animals. In fact, we do not see again in their hearts, which have only one ventricle, the structure which really belongs to the animals of the first and second ranks. Their blood is cold, almost like that of the animals of the later ranks.

Another proof of the degradation in organic structure of reptiles is presented to us in their respiration. First of all, these are the last animals to breathe by means of a real lung. For, after them, we do not see again in any of the animals in the following classes a respiratory organ of this sort (something I will try to establish in discussing the mollusks). Then, with the reptiles, the lung, in general, has very large chambers, proportionately fewer in number and already much simplified. In many species, this organ is missing in the early ages and finds itself then replaced by gills, a respiratory organ which we never find in the previous ranks. Sometimes here we find the two sorts of respiratory organs mentioned simultaneously in the same individual.

But the greatest proof of the degradation concerning the respiration of reptiles is that there is only one part of their blood which goes through the lung, while the rest reaches the parts of the body without having received the influence of respiration.

Finally, with the reptiles, the four limbs essential to the most perfect animals begin to be lost, and many of them (almost all the snakes) even completely lack them.

Apart from the degradation in the organic structure acknowledged in the form of the heart, the temperature of the blood, which is only just above that outside in the environmental surroundings, the incomplete respiration, the almost gradual simplification of the lung, we notice that the reptiles differ considerably among themselves. The result is that the animals in each of orders of this class display greater differences in their organic structure and in their exterior form than those of the two preceding classes. Some live habitually in the open air, and among them, those which do not have legs can only crawl. Others live in water or at the water's edge, moving back, sometimes into the water and sometimes into open places. There are some which are covered in scales and others which have a bare skin. Finally, although they all have a heart with one ventricle, in some there are two auricles and in others only one. All these differences are relevant to environmental circumstances, ways of life, and so on, circumstances which, undoubtedly, have a more powerful influence on an organic structure which is still some way from the goal to which nature tends. These circumstances were not able to exert such an influence on those animals more advanced in their improvement.

Thus, the reptiles are oviparous animals (even those whose eggs are enclosed in the womb of the mother), have a modified skeleton, very often very degraded, display a respiration and a circulation less perfected than those of mammalian animals and birds, and all manifest a small brain which does not completely fill the skull cavity. Thus, they are less perfect than the animals of the two preceding classes and confirm, on their part, the increasing degradation in organic structure, as we move closer to the most imperfect animals.

Among these animals, apart from the modifications to the shape of their parts resulting from the circumstances in which they live, we notice, in addition, traces of the general degradation in organic structure. For in the last of their orders (in the batrachians), individuals in the early stages breathe by gills.

If we consider the lack of legs (which we see in the snakes) a consequence of degradation, then the ophidians would make up the last order of reptiles. But to accept this idea would be a mistake. In fact, the snakes are animals which, in order to conceal themselves, have acquired the habit of crawling right on the ground. Their bodies have attained a considerable length disproportionate to their size. Now, the long legs would have hindered their need to crawl and hide, and very short limbs, which could only have been four in number (since these are vertebrate animals) would not have been capable of moving the body. Thus, the habits of these animals made their limbs disappear. Nonetheless, the batrachians, which do have legs, display a more degraded organic structure and are closer to fish.

The proofs for the important idea I am setting down will be established with reliable facts. Consequently, they will always be beyond the challenges which people might like in vain to offer in opposition to them.


Animals breathing through gills, having a smooth skin or covered with scales and the body furnished with fins.

In following the course of this degradation in organic structure and in the number of faculties manifested in the total collection of animals, we see that fish must necessarily be placed in the fourth rank, that is, after the reptiles. They have, in fact, an organic structure still less perfected than that of the reptiles, and thus, more distant from that of the most perfect animals.

Without doubt, their general shape, lack of a narrowing between the head and the body (to form a neck), and the different fins serving them in place of limbs result from the influence of the dense milieu in which they live and not from the degradation in their organic structure. But this degradation is no less real and impressive, as we can see by examining their interior organs. That structure is such that it forces us to assign fish a rank after that of the reptiles.

In fish we do not find any more the respiratory organs of the most perfect animals; that is, fish lack a true lung and in place of this organ have only gills or vascular pectinate folds, placed on the two sides of the neck or the head, a total of four on each side. The water which these animals take in by the mouth passes between the layers of gills, bathing the numerous blood vessels located there. Since this water is mixed with air or contains air in solution, the air, although in small amounts, works on the blood in the gills and brings about the benefits of respiration. Then the water goes out sideways through the gills, that is, by the holes opened up on the two sides of the neck.

Now, note that this is the last time that a respiratory fluid will enter by the animal's mouth to reach the respiratory organ.

These animals, as well as those of the later ranks, do not have a trachea, a larynx, a real voice (even those called grondeurs), eyelids on their eyes, and so on. Here we see organs and faculties lost, ones which we do not find again any more in the rest of the animal kingdom.

However, the fish still are a part of the group of vertebrate animals. But they are the last of it, and they bring to a close the fifth level of organic structure, being, along with the reptiles, the only animals which have the following:

--a vertebral column;
--nerves ending in a brain which does not fill up the skull;
--a heart with one ventricle;
--cold blood;
--finally, an entirely internal ear.

In addition, the fish display in their organic structure an oviparous reproductive system. They have a body without mammary glands, a body shaped very appropriately for swimming, fins which are not completely analogous to the four limbs of the most perfect animals, a very incomplete and remarkably modified skeleton (barely outlined in the last animals of this class), a single ventricle in the heart, cold blood, gills in place of a lung, a very small brain, a sense of touch incapable of understanding the shape of bodies, and apparently without a sense of smell, because odours are transmitted only through the air. It is obvious that these animals, for their part, strongly confirm the degradation in organic structure which we have undertaken to follow through the full extent of the animal kingdom.

Now we are going to see that the main division of fish shows us, in the fish called bony, the most improved among them, and in the fish called cartilaginous, the least improved. These two points confirm, within this very class, the degradation in the organic structure. For the cartilaginous fish with their soft cartilaginous parts designed to make their bodies stronger and to make their movements easier show us that among them the skeleton ends, or rather that with them nature started to sketch out the skeleton.

As we still follow the order in the direction opposite to that followed by nature, the eight final genera of this class must include the fish whose brachial openings, without an operculum or a membrane, are only holes on the side or under the throat. Finally, the lampreys and the hag fish must come at the end of the class, because these fish are extremely different from all the others on account of the imperfection of their skeletons, and their bare viscous bodies, lacking lateral fins, and so on.

Observations on the Vertebrates

Although the vertebrate animals manifest among themselves great differences in their organs, all their organic structures appear to be formed on a common plan. As we go up from the fish to the mammals, we see that this plan has been improved from class to class and that it has stopped completely only in the most perfect mammals. But we also notice that in the course of its improvement, this plan has undergone numerous modifications, some even very significant, on account of the influence of the environmental conditions of the animals, as well as of the habits which each race has been compelled to acquire according to the circumstances in which it found itself.

Hence, we notice, on the one hand, that if the vertebrate animals are very different from one another in the condition of their organic structure, the reason is that nature began to carry out her plan for them only in the fish, later improved it in the reptiles, brought it closer to its completion in the birds, and finally succeeded in bringing it completely to a conclusion only in the most perfect mammals.

On the other hand, we cannot avoid the recognition that, if the improvements in the planned organic structure in vertebrates do not illustrate everywhere, from the most imperfect fish right up to the most perfect mammals, a regular modulated gradation, the reason is that the work of nature has often been changed, thwarted, and even turned away from its direction, by the influences which remarkably different, even contrasting circumstances have exerted on the animals exposed to them in the course of a long succession of generations.

Disappearance of the Vertebral Column

At this point of the animal ladder, we find the vertebral column completely done away with. Since this column is the basis for all true skeletons and since this framework of bone constitutes an important part of the organic structure of the most perfect animals, all the invertebrate animals we are going to examine in succession thus have an organic structure even more degraded than it is in the four classes which we have just reviewed. Also from now on, the supports for muscular action will not be placed any more on interior parts.

Moreover, none of the invertebrate animals breathes by lungs with chambers or has a voice (and thus an organ for this faculty). Finally, they appear, for the most part, to lack real blood, that is, the essentially red fluid in the vertebrates which derives its colour only from the intensity of its animalisation and, above all, which establishes a real circulation. Surely it would be an abuse of language to give the word blood to the colourless thin fluid which moves slowly in the cellular substance of polyps? Will it then be necessary to assign a similar name to the sap in plants?

In addition to the vertebral column, in invertebrates is also lost the iris, which characterizes the eyes of the most perfect animals. For, among the invertebrate animals, those with eyes do not have them clearly equipped with irises.

Similarly, the kidneys are found only in the vertebrate animals, fish being the last ones in which we still encounter this organ. From here on, there is no more spinal marrow, no more large sympathetic nerve system.

Finally, a very important observation to note is that in the vertebrates, and mainly towards the end of the animal scale which displays the most perfect animals, all the essential organs are isolated, or each one has an isolated location in as many particular spots. We will soon see that the reverse of this has taken place, as we move towards the other end of the same scale.

It is thus evident that the invertebrate animals all have a less perfect organic structure than those which possess a vertebral column; the organic structure of mammals displays a vertebral column, a feature which indicates the most perfect animals with respect to all the affinities, and is, without dispute, the true pattern of the most perfect organic structure.

Let us see now if the classes and the large families which are part of the numerous series of invertebrate animals display also, in a comparison of their groups, an increasing degradation in the complexity and the perfection of the organic structure in the animals which constitute them.


When we reach invertebrate animals, we enter an immense series of different animals, the most numerous in nature, the most curious and the most interesting with respect to the affinities in the observable differences of organic structure and faculties.

One is convinced, by observing their condition, that to bring them successively into existence, nature proceeded gradually from simplest to the most complex. Now, since nature intended to reach a planned organic structure which would permit the greatest improvement (that of the vertebrate animals), a plan very different from the one which she was probably forced to create to reach that goal, one senses that among these numerous animals we should run into not a single system in which the organic structure is improved progressively but various very distinct systems, each of which must have come about at the point where each organ of primary importance started to come into being.

In fact, when nature succeeds in creating a special organ for digestion (as in the polyps), for the first time she gave a particular and constant form to the animal furnished with it. The infusorians, where she started it all, could not possess either the faculty which this organ provides or the style and shape of its organic structure in order to develop its functions.

Later, when nature established a special organ for respiration, as she varied this organ to improve it and to accommodate it to the environmental circumstances of the animals, she diversified the organic structure according to what the existence and the development of the other special organs successively required.

When, after that, nature succeeded in producing the nervous system, it was at once possible to create a system of muscles, and from then on it was necessary to have strong points of attachment for the muscles, the paired parts making up a symmetric form. The result of that was different types of organic structure, because of the environmental circumstances and the acquired parts, which could not have arisen previously.

Finally, when nature had attained sufficient movement in the fluids contained by the animal, so that the circulation could be organized, that once again resulted in important particular features in the organic structure which distinguishes it from organic systems in which circulation does not occur at all.

To notice the basis of what I have just laid out and to set out evidence for the degradation and the simplification in the organic structure (since we are following the order of nature in the reverse direction), let us run quickly through the different classes of invertebrate animals.


As one goes down the graduated ladder which forms the series of animals, the fifth rank necessarily belongs to the mollusks. For although we place them a degree lower than the fish, because they have no vertebral column, these are nevertheless the best organized of the invertebrates. The breathe by gills, but very diversified ones, in their form and size, or in their location on the inside or the outside of the animal, according to the genera and the habits of the races which these genera include. They all have a brain, nerves which do not have ganglia, that is, which do not display a row of ganglions along the length of a longitudinal marrow, arteries and veins, and one or a several one-chambered hearts. These are the only known animals which possess a nervous system but not a spinal column or a longitudinal marrow with ganglia..

The gills basically destined by nature to make respiration work in the very depths of the sea had to undergo modifications in relation to their faculties and their forms in the aquatic animals who, in the course of reproducing individuals of their race, went out often to expose themselves to the air and, as with many of these races, to remain there habitually.

The respiratory organ of these animals became imperceptibly accustomed to the air. This is not a supposition. For we known that all the crustaceans have gills; however, we know of crabs (cancer ruricola) who live habitually on the earth, breathing the natural air with their gills. Finally, this habit of breathing air with gills became necessary for plenty of mollusks who acquired it. The habit modified the organ itself, so that the gills of these animals did not require any more so many points of contact with the respiratory fluid and became stuck to the inner walls of the cavity which contains them.

As a result, we distinguish among the mollusks two types of gills.

Some are made up of a maze of vessels which move up onto the skin of an interior cavity which does not form any projection and which can only breathe air. We can call these air gills.

The others are the organs almost always projecting, inside or outside the animal, forming fringes or pectinate layers or thin cords, and so on, which cannot make respiration occur except with the assistance of fluid water. We can call these aquatic gills.

If differences in the habits of the animals have brought about changes in their organs, we can at this point conclude from this fact that, for the study of characteristics peculiar to certain orders of mollusks, it will be useful to distinguish those who have air gills from those whose gills cannot respire except in the water. But one way or the other, it is always a matter of gills, and it seems to us very inconvenient to state that the mollusks which breathe air possess a lung. Who is not aware how often the abuse of words and the false application of names have helped to distort objects and to throw us into error?

Is there such a great difference between the respiratory organ of the Pneumoderma, which consists of a maze or vascular threads spread over the top of an exterior skin and the vascular maze of snails which spreads across an interior skin. The Pneumoderma, however, appears to breath only water.

Besides, let us examine for a moment whether there are affinities between the respiratory organ of mollusks which breathe air and the lungs of vertebrate animals.

The characteristic property of the lung is to consist of a specific spongy mass, composed of more or less numerous cells, in which the natural air always moves, at first through the mouth of the animal, and from there through a more or less cartilaginous canal, called the trachea, which generally is divided into networks called bronchia, which end up in the cells. The cells and the bronchia are alternately filled with and emptied of air as a result of the successive swelling and collapse of the bodily cavity containing the mass of the organ. Consequently, it is a specific feature of the lung that it presents distinct and alternating inhalations and exhalations. This organ can survive only with direct contact with the air and becomes powerfully irritated by contact with water or any other material. It is thus of a different nature from the branchial cavity of certain mollusks which is always unique, which displays no distinct inhalation and exhalation and no alternating swelling and collapsing, which never has a trachea or bronchi, and in which the respiratory fluid never enters by the animal's mouth.

A respiratory cavity which displays neither trachea nor bronchi and no alternate swelling and collapsing, in which the respiratory fluid does not enter by the mouth, and which adapts itself, sometimes to the air, sometimes to the water, would not be capable of being a lung. To mix such different objects up with the same name is not to advance science but to embarrass it.

The lung is the only respiratory organ which can give an animal the vocal faculty. After the reptiles, no animal has a lung; hence, none has a voice.

I conclude that it is not true that there are mollusks which breathe by a lung. If some breathe natural air, certain crustaceans do the same, as well as all the insects. But none of these animals has a true lung, so long as we do not give the same name to very different objects.

If the mollusks, because of their general organic structure, inferior in its improvements compared to fish, also establish, for their part, the progressive degradation which we are examining in the animal chain, the same degradation among the mollusks themselves is not so easy to determine. For among the very numerous and very diversified animals of this class, it is difficult to separate what belongs to the degradation in question and what is the product of the places where these animals live.

In truth, the two unique orders dividing the numerous class of mollusks very clearly contrast with each other in the importance of their distinct characteristics. The animals of the first of these orders (the cephalid mollusks) have a very distinct head, eyes, jaws or a proboscis, and reproduce by mating.

By contrast, all the mollusks of the second order (the acephalid mollusks) have no head, eyes, jaws, oral proboscis, and never reproduce by mating.

Now, one cannot deny that the second order of mollusks is inferior to the first with respect to an improved organic structure.

However, it is important to consider that the lack of head, eyes, and so on, in the acephalid mollusks is not uniquely the result of the general degradation in organic structure, since in the lower ranks of the animal chain, we find once more animals with a head, eyes, and so on. But it seems here that this is one of those deviations in the progressive improvements in organic structure produced by circumstances and, consequently, by causes foreign to those which gradually increase the complexity of organic structure in animals.

In considering the influence of the use of organs (the influence of an absolute absence of use and of constant use), we will see, in fact, that a head, eyes, and so on would have been particularly useless to the mollusks of the second order, because the great development in their mantle would not have permitted these organs any use whatsoever.

In conformity with that natural law which wills that all organs continually unused imperceptibly deteriorate, wither up, and finally disappear completely, we find the head, eyes, jaws, and so on, done away with in the acephalid mollusks. We will see elsewhere plenty of other examples of this.

In the invertebrate animals, since nature did not find in the interior parts points of attachment for muscular movement, she substituted, in the mollusks, the mantle with which she has furnished them. Now, this mantle in mollusks is stiffer and more compressed, the more these animals move about and the more they have only this mantle to assist them.

Thus, in the cephalid mollusks, where there is more movement than in those who do not have a head, the mantle is narrower, thicker, and stiffer. Among these cephalid mollusks, those which are bare (without shells) have in their mantle an additional breast plate even stiffer than the mantle itself, which makes the movements and the contractions of the animal (slugs) considerably easier.

But if instead of following the animal chain in the reverse order to nature's, we move through it from the most unimproved to the most improved, then it would be easily perceived that nature, just at the point of starting the plan of the organic structure of vertebrate animals, was forced, in the mollusks, to abandon the use of a shell or horny covering for the points of attachment for muscular action and that, as nature prepared to bring these points of attachment into the interior of the animal, the mollusks were placed, in some way, at the transition point in the system of organic structure. Consequently, having nothing more than feeble means of moving around, they execute these movements only remarkably slowly.


Animals without eyes, breathing by gills, furnished with a mantle and articulated arms with a horny skin.

The cirrhipedes, for whom we as yet know only four genera (2) must be considered as forming a special class, because these animals cannot be included in the group of any other class of invertebrate animals.

They are like the mollusks on account of their mantle, and one has to place them immediately after the acephalid mollusks, since, like them, they lack heads and eyes.

However, the cirrhipides cannot be included in the class of mollusks. For their nervous system displays, like the animals of the three classes which follow, a longitudinal marrow with ganglia. In addition, they have articulated arms with a horny skin and several pairs of transverse jaws. Thus, they are in a rank below that of the mollusks. The movements of their fluids takes place through a real circulation, with the help of arteries and veins.

These animals are established on marine bodies and consequently do not move around. Thus, their main movements are limited to those of their arms. Now, although they have a mantle like the mollusks, nature could not gain any help for movements of their arms from that and was forced to create on the skin of these arms points of attachment for the muscles which must move them. Thus, this skin is tough and rather horny, like the skin of crustaceans and insects.


Animals with elongated and ringed bodies, without articulated limbs, breathing by gills, and with a circulatory system and a longitudinal marrow with ganglia.

The class of annelids comes of necessity after the class of cirrhipedes, because no annelid has a mantle. We are also forced to place them before the crustaceans because these animals do not have articulated limbs, because they must not interrupt the series of those who have them, and because their organic structure does not permit us to assign them a rank after the insects.

Although these animals are, in general, as yet very little understood, the rank which their organic structure gives them proves that, so far as they are concerned, the degradation in organic structure continues to maintain itself. For, from this point of view, they are inferior to the mollusks, having a longitudinal marrow with ganglia. Moreover, they are also inferior to the cirrhipedes who have a mantle like the mollusks, and their lack of articulated limbs does not permit one to put them in a position where they break up the series of those manifesting this organic structure.

The elongated shape of the annelids, which they owe to their ways of living, whether buried in the damp earth or silt, or in the waters, where they live, for the most part, in tubes of different materials, which they leave and return to as they wish, makes them so like worms that all the naturalists up to this point have confused them with worms.

Their interior organic structure displays a very small brain, a longitudinal marrow with ganglia, some arteries and veins in which circulates a blood usually coloured red. They breathe through gills, sometimes external ones which project and sometimes internal ones hidden or not apparent.


Animals having a body and articulated limbs, a crustaceous skin, a circulation system, and breathing by gills.

At this point we enter into the numerous series of animals in which the body and especially the limbs are articulated and the integuments are crustaceous, horny, or coriaceous.

The solid or strong parts of these animals are all on the outside. Now, since nature created the muscular system very shortly before the first animals of this series and required points of attachment in solid parts to give the system energy, she was obliged to establish the method of articulations to make movement possible.

All the animals united by a similar method of articulation have been considered by Linnaeus and, following him, as forming only one single class, to which was given the name insects. But we finally recognized that this large series of animals displays several important divisions which it is essential to distinguish.

Also, the class of crustaceans, which has been confused with the class of insects (although all the ancient naturalists) had always separated it, is a division indicated by nature and essential to preserve. It must follow immediately after the annelids and occupy the eighth rank in the general series of animals. The analysis of its organic structure requires that. There is nothing at all arbitrary in this matter.

In fact, the crustaceans have a heart, arteries and veins, a transparent, almost colourless circulating fluid, and all breathe by true gills. That is incontestable and will always embarrass those who continue to rank them among the insects because they have articulated limbs.

If, because of their circulation and the respiratory organs, the crustaceans are clearly distinguished from the arachnids and the insects and if, consequently, their rank is clearly superior, nevertheless they share with the arachnids and insects this lower feature of organic structure, with respect to the annelids, that is, they are part of the series of animals with articulated limbs, a series in which we see the circulation system going away and disappearing. Consequently, the heart, the arteries and veins, and even the respiration by the system of gills similarly is lost. Thus, the crustaceans confirm, in their turn, the degradation maintained in the organic structure, in the direction we are moving through the animal ladder. The fluid which circulates in their vessels is transparent and almost without substance, like that in the insects, and demonstrates once more this degradation with respect to them.

As to their nervous system, it consists of a very small brain and a longitudinal marrow with ganglia, a characteristic impoverishment of this system, which we see in the animals of the two preceding classes and the two which follow, for the animals of these classes are the last ones in which the nervous system is still present.

It is in the crustaceans that the last traces of the gill organ has been perceived. After them, it does not reappear in any animal.


At this point the existence of a true system of circulation comes to an end, that is, a system of arteries and veins which makes up part of the organic structure of the most improved animals and which all the structures of all the preceding classes of animals possess. The organic structure of the animals which we are going to discuss is thus more imperfect still that that of the crustaceans, who are the last in which the circulation is well manifested. Thus, the degradation in the organic structure continues in a clear manner, because to the further one moves ahead in the series of animals, all the features of resemblance between the organic structures of the animals we are considering and that of the most improved animals is successively lost.

Whatever the nature of the movement of fluids in the animals of the classes which we are going to go through, this movement works by less active means and always in a slower way.


Animals breathing by narrow trachea, not undergoing any transformation, and having at all times articulated limbs and eyes in their heads.

In continuing the order which we have followed up to the present, the ninth rank in the animal kingdom necessarily belongs to the arachnids. They have so many affinities with the crustaceans, that we will always be compelled to bring them close together and to place one immediately after the other. Nevertheless, they are clearly distinguished from the crustaceans. For they display the first example of a respiratory organ inferior to gills, because we do not ever come across it in the animals which have a heart, arteries, and veins.

In fact, the arachnids breathe only by stigmata and air trachea which are respiratory organs analogous to those of insects. But these trachea, instead of extending themselves throughout the entire body, like those in insects, are circumscribed in a small number of vesicles. This fact shows that nature concludes, in the arachnids, the method of respiration which she had to use before establishing gills, just as she concluded, in the fish or in the last reptiles, what she had had to make sure of before she could form a true lung.

If the arachnids are clearly distinguished from the crustaceans, because they do not breathe at all by gills but by very narrow air trachea, they are also clearly different from the insects. It would be just as inconvenient to group them with the insects, whose classic characteristic they do not have, and from whom they differ especially in their interior organic structure as to mix up the crustaceans with the insects.

In effect, the arachnids, although having some important similarities to insects, are essentially distinct from them, for the following reasons:

(1) They never undergo a metamorphosis. They are born with the shape and all the parts which they must always retain and, consequently, they always have eyes in the head, articulated limbs, things arising out of the nature of their interior organic structure. In this they are very different from the organic structure of insects.

(2) In the arachnids of the first order (pedipalp-arachnids) we begin to notice the traces of a system of circulation (3).

(3) With respect to their respiratory system, although of the same order as that of the insects, it is nevertheless very different, because their trachea, limited to a small number of vesicles, are not made up of very numerous canals to the air extending throughout the animals' bodies, as we see with the trachea of insects.

(4) Finally, the arachnids reproduce several times in the course of their lives, an ability which the insects lack.

These considerations should suffice to enable us to sense how faulty those distributions are which combine arachnids and insects in the same class, because their authors have taken into account only the articulations of the limbs of these animals and the more or less crustaceous skin covering them. That is a rather as if we took into account only the fact of the more or less scaly teguments of the reptiles and fish and thus combined them in the same class.

As for the general degradation in organic structure which we are looking into as we move through the entire animal scale, that is very clearly evident in the arachnids. Since these animals actually breathe through an organ less well developed with respect to structural improvements than the lung and even gills and have only the preliminary traces of a circulation which does not yet appear complete, they confirm, on their part, the continuing degradation in question.

This degradation is noticed even in the series of species brought together in this class. For the arachnids with antennae or of the second order are very different from others, are very inferior to them in the development of their organic structure and are very close to insects. Nonetheless, they differ from insects in that they do not undergo any transformation. Since they never fly up into the air, it is very probable that their trachea do not generally extend to all parts of their bodies.


Animals undergoing transformations of form and having, in the perfect state, two eyes and two antennae on the head, six articulated limbs, and two trachea extending throughout the entire body.

As we continue to follow the inverse order to nature's, after the arachnids necessarily come the insects, that is, this immense series of imperfect animals which have neither arteries, nor veins, which breathe by air trachea which are not limited, and finally which, being born in a state less perfect than that in which they reproduce, consequently undergo metamorphosis.

Once they reach their perfect condition, all the insects, without exception, have six articulated limbs, two antennae and two eyes on the head, and most of them then have wings.

According to the order we are following, the insects of necessity take up the tenth rank in the animal kingdom. For they are inferior in improvements to their organic structure to the arachnids, because they are not, like the latter, born in their perfect condition and they reproduce only once during the course of their lives.

Particularly among the insects we begin to notice that the organs essential to maintain life are distributed almost equally and most of them are situated throughout the extent of the body, rather than being isolated in particular places, as was the case in the most improved animals. This consideration gradually loses its exceptions and becomes more and more striking in the animals in the later classes.

Nowhere, up to this point, is the general degradation in organic structure more manifest than in the insects, where that structure is inferior in improvements to that of the animals in all the preceding classes. This degradation even shows up between the different orders which naturally divide the insects. For those in the three first orders (Coleoptera, Orthoptera, and Neuroptera) have mandibles and jaws in their mouths. Those of the fourth order (Hymenoptera) begin to possess a sort of proboscis; finally those of the four last orders (Lepidopteres, Hemiptera, Diptera and Aptera) really have only a proboscis. Now, paired jaws do not turn up anywhere in the animal kingdom after the insects of the three first orders. With respect to wings, the insects of the six first orders have four of them, of which all or two alone serve for flight. Those of the seventh and eighth orders have only two wings, or their wings are aborted. The larvae of insects of the two final orders do have limbs and resemble worms.

It appears that insects are the last animals which display a clearly distinct sexual reproduction and which are truly oviparous.

Finally, we will see that the insects are infinitely remarkable for the details concerning what has been called their industry but that this alleged industry is not at all the product of any thinking, that is, of any combination of ideas on their part.


Just as the fish, among the vertebrates, display in their general shape and in the anomalies relative to the progression in complexity of organic structure, the results of the influence of their environmental habitat, so the insects, among the invertebrates, display in their form, organic structure, and transformations, the clear result of the influence of the open air in which they live and up into which most of them leap and habitually stay, like the birds.

If the insects had had a lung, if they had been capable of inflating themselves with air, and if the air which reaches all parts of their body had been able to rarefy itself there, like the air which goes into the bodies of birds, their hairs would have been changed, no doubt, into feathers.

Finally, if, among the invertebrate animals, it is astonishing to find so few affinities between the insects which undergo singular metamorphoses and the invertebrate animals of the other classes, one should draw attention to the fact that these are the only animals without vertebrae who leap up into the air and carry out progressive movements there. Then one will sense that quite special circumstances and habits must have produced results which are just as special.

The insects are close only to the arachnids in their interrelationships. And, in fact, both of them are, in general, the only animals without vertebrae which live in the air. But no spider has the ability to fly, and no arachnid likewise undergoes transformation. In dealing with influences of habits, I will show that these animals, because they were accustomed to stay on parts of the surface of the earth and to live in hidden places, must have lost some of the ability of insects and to acquire characteristics which clearly distinguish them from insects.

The Destruction of Several Organs Essential to the More Improved Animals

After the insects, it appears that there is quite a considerable gap in the series, which animals we have not observed remain to fill in. For in this position in the series, several organs essential to more improved animals suddenly are missing and are really done away with, because we do not find them again in the classes which remain for us to review.

The Disappearance of the Nervous System

Here the nervous system (the nerves and their central connection) effectively disappears completely and does not appear any more in any of the animals of the classes which follow.

In the most improved animals, this system consists of a brain which appears to serve to carry out of acts of intelligence, at the base of which is located the chamber of sensations, from which the nerves leave, as well as a dorsal spinal chord which sends other nerves out to the various parts.

In the vertebrates, the brain successively diminishes, and as its volume gets smaller, the spinal column becomes larger and seems to take its place.

In the mollusks, the first class of invertebrates, the brain still exists, but it does not have a spinal chord or longitudinal marrow with ganglia. Since the ganglia are rare, the nerves do not appear knotty.

Finally, in the five classes which follow, the nervous system, in its last stage, is reduced to a very small brain, hardly a trace, and a longitudinal marrow which sends nerves to the parts. From that point on there is no isolated chamber for sensations, but a multitude of small places spread throughout the full length of the animal.

Thus, in the insects the important system of feeling ends, that system which, at a particular stage of its development, gives rise to ideas, which in its highest perfection can produce all intelligent acts, and which, finally, is the source where muscular action gets its power and without which sexual reproduction apparently cannot exist.

Disappearance of the Sexual Organs

Again, here traces of sexual reproduction disappear completely and, in fact, in the animals which are going to be listed, it is no longer possible to recognize the organs of a true fertilization. Nevertheless, we are going to find again in the animals of the two classes which follow, species with types of ovaries in oviform corpuscles which are taken for eggs. But I consider these alleged eggs, which can produce without prior fertilization as buds or internal gemmules. They create the link between internal gemmiparous reproduction and oviparous sexual reproduction.

The inclination of human beings for their own habits is so great that they persist, even against the evidence, to look at things always in the same manner.

This is the reason why botanists, accustomed to observe the sexual organs of a large number of plants, want all plants, without exception, to have similar organs. Consequently, several of them have made every imaginable effort, with respect to the plants cryptogames or agames, to discover stamens and pistils. And they have preferred to attribute to the plants arbitrarily and without proof functions to those parts whose use they did not know rather than to recognize that nature knows how to reach the same goal by different means.

We were persuaded that every reproductive body is a seed or an egg, that is, a body which, in order to be reproductive, needs to undergo the influence of sexual fertilization. This point made Linnaeus state: Omne vivum ex ovo [All living things are from the egg]. But we now know very well plants and animals which regenerate themselves solely by bodies which are neither seed nor eggs and which, consequently, have no need for sexual fertilization. Thus, these bodies are shaped differently and develop in a different manner.

Here is the principle to which we must pay attention in order to assess the method of reproduction of any living body whatever.

Every reproductive corpuscle, whether plant or animal, which, without breaking through any enclosing material, grows longer and larger and become a plant or animal similar to the one which it came from is neither a seed nor an egg. It does not undergo any fertilization or hatch after having started to grow, and its formation has not required any sexual impregnation. Thus it does not contain an embryo enclosed in layers which must be broken through, as in the seed or the egg.

Now, follow attentively the developments of the reproductive corpuscles of algae, fungi, and so on, and you will see that these corpuscles only grow longer and larger so as to take on imperceptibly the form of the plant from which they issued forth. They do no break through any enclosing material, as does the embryo of the seed or the one that contains the egg.

Similarly, if you follow the gemma or bud of a polyp, like the hydra, you will be convinced that this reproductive body only grows longer and larger, that it does not break through any exterior covering, in a word, that it does not hatch like a chicken or a silk worm coming from an egg.

Thus, it is clear that all reproduction of individuals does not take place by way of sexual fertilization and that where sexual fertilization does not bring it about, there is essentially no true sexual organ. Now, since, after the insects, we do not distinguish in the four classes of animals which follow any organ of fertilization, it appears that it is at this point in the animal chain that sexual reproduction ceases to exist.

Disappearance of the Organ for Sight

Here again, the organ for sight, so useful to the most perfect animals, is entirely done away with. This organ, which started to be absent in a section of the mollusks, in the cirrhipedes, and in most of the annelids, and which is found again in the crustaceans, arachnids, and insects only in a very imperfect condition, with an extremely limited use (or none at all) does not reappear after the insects in any animal.

Finally, at this point also the head, that essential body part in the most perfect animals, the seat of the brain and almost all the senses, totally ceases to exist. For the bulge in the forward extremity of the body of some worms, like the Taenia, which is caused by the arrangement of their suckers, is neither the seat of a brain nor of organs of hearing, sight, and so on, because all these organs are lacking in the animals of the classes which follow; the swelling in question cannot be considered a true head.

We see that at this stage of the animal scale, the degradation in organic structure becomes extremely rapid and that it makes one look ahead to the greatest simplification in the organic structure of animals.


Animals with soft, long bodies, without a head, eyes, articulated limbs, lacking a longitudinal marrow and a system of circulation.

We are here considering the worms, who do not have any vessels for circulation, for instance, those which we know about under the name intestinal worms, and some other non-intestinal worms, whose organic structure is just as imperfect. These are animals with a soft body, more or less elongated, not undergoing any metamorphosis, and all lacking head, eyes, and articulated limbs.

The worms must immediately follow the insects, come before the radiates, and occupy the eleventh rank in the animal kingdom. Among them we see starting nature's tendency to establish the system of articulations, a system which she has later completely developed in the insects, arachnids, and crustaceans. But the organic structure of worms is less perfect than that of insects, because they no longer have a longitudinal marrow, head, eyes, and real limbs. Thus, this organic structure compels us to put them after the insects. Finally, the new style in the shape which nature starts in them to establish a system of articulations and to move away from the radiating system in the parts proves that we must place the worms before the radiates themselves. In addition, after the insects we lose the plan carried out by nature in the preceding classes, that is, this general animal shape which consists of an opposing symmetry in the parts, in such a way that each of the parts is opposite an entirely similar part.

In the worms, we no longer find this opposing symmetry in the parts, and we do not see again the radiating arrangement of the organs, both interior and exterior, which we notice in the radiates.

Since the time that I established the class of annelids, some naturalists have given the name worms to annelids themselves. Since they then did not know what to do about the animals in question, they combined them with the polyps. I leave it to the reader to judge what affinities and classic characteristics authorize us to combine in the same class a Tenia or an Ascaris with a Hydra or any other polyp.

Like the insects, several worms appear still to breathe by trachea, whose openings to the outside are s type of stigmata. But there is reason to believe that these limited and imperfect trachea are aquatic and not aerial, like those of insects, because these animals never live in the open air and are always either deep in water or bathed in the fluids which contain them.

No organ of fertilization is very distinct in them. Thus, I assume that sexual reproduction has no role with this animal. Nevertheless, it could be possible that, just as the circulation is hinted at in the arachnids, so sexual reproduction is sketched out in the worms. This seems to be indicated by the different shapes of the tail in the Strongylus. But observation has not yet well established this reproduction in these animals.

What we see in some of the worms and what we take to be ovaries (as in the Tenia) appear to be only a mass of reproductive corpuscles which do not require fertilization. These oviform corpuscles are internal, like those in the sea urchin, instead of being external like those in the Coryne, and so on. The polyps display amongst themselves the same differences with respect to the position of the gemmules which they produce. Thus, it is plausible that the worms are internally gemmiparous.

Some animals which, like the worms, lack head, eyes, limbs and perhaps sexual reproduction, thus also establish, in their turn, the degradation maintained in the organic structure which we are looking into in the entire extent of the animal scale.


Animals with a regenerating body, lacking a head, eyes, articulated limbs, with a mouth on the under surface, and a radiating arrangement in their internal and external parts.

Following the customary order, the radiates occupy the twelfth rank in the numerous series of known animals and make up one of the three last classes of invertebrate animals.

When we arrive at this class, we encounter in the animals included in it a style of general shape and arrangement of parts and organs, both external and internal, which nature has not used in any animals of the previous classes.

In effect, the radiates very clearly have in their internal and external parts a radiating arrangement around a centre or an axis which makes up a special shape which nature has not, up to that point, ever used. She began to sketch out such an arrangement only in the polyps, which, consequently, come after the radiates.

Nevertheless, the radiates make up in the animal scale a compartment very different from that of the polyps, so that it is no longer possible to confuse the radiates with the polyps any more than it is to group the crustaceans with the insects or the reptiles among the fish.

In fact, in the radiates not only do we still observe organs which appear destined for respiration (tubes or types of aquatic trachea), but we also see, in addition, special organs for reproduction, like types of ovaries in various shapes. Nothing similar is found in the polyps. Moreover, the intestinal canal of radiates is not generally a closed tube with only one opening, as in all the polyps. And the mouth, always low down or on the lower surface, manifests in these animals a special arrangement which is not at all the one which the polyps display in their general structure.

Although the radiates are truly curious and as yet little understood animals, what we know about their organic structure clearly indicates the rank to which I am assigning them. Like worms, the radiates are headless, eyeless, without articulated limbs, a system of circulation, and perhaps without nerves. However, the radiates come necessarily after the worms. For the latter have nothing in the arrangement of their interior organs which tends to a radiating shape, and it is among them that the style of articulations begins.

If the radiates lack nerves, they are then without the faculty of feeling and are no more than merely irritable. This seems to be confirmed by observations made on living star fish in which the arms have been cut off without their showing any sign of pain.

In many radiates fibres are still distinct. But can we call these fibres muscles, unless we are justified in stating that a muscle deprived of nerves is still capable of carrying out its functions? Do we not have, in plants, the example of the potential possessed by cellular tissue of being reduced to fibres without our being able to consider these fibres muscles? Every living body in which we make out fibres does not, it seems to me, have muscles just for this reason. And I think that where there are no more nerves, the muscular system no longer exists. There is reason to believe that in animals without nerves the fibres which we can still come across there possess, through their simple irritability, the faculty of producing movements which replace muscular movement, although with less energy.

In the radiates, not only does it appear that the muscular system no longer exists, but also that there is no more sexual reproduction. In fact, nothing gives evidence of that or even indicates that the small oviform bodies, whose mass make up what we call the ovaries of these animals, undergo fertilization (and are thus true eggs). This is all the more implausible when we find them equally in every individual. Thus, I consider these small oviform bodies to be internal gemmules already perfected, and their clumping together in special places is the method nature has prepared to arrive at sexual reproduction.

The radiates, confirm, in their turn, the general degradation in the organic structure of animals. For in this class of animals, we meet a new form and arrangement of parts and organs which are far distant from those of animals in the preceding classes. Moreover, they appear to lack feeling, muscular movement, and sexual reproduction. Among them, we see the intestinal canal cease to have two openings, the clusters of oviform corpuscles disappear, and the bodies become entirely gelatinous.


It seems that in the very imperfect animals, like the polyps and the radiates, the centre of movement of the fluids no longer exists except in the alimentary canal. There it commences to establish itself, and by the way of this canal the subtle ambient fluids mainly enter to stimulate movement in the containing fluids appropriate to these animals. What would plant life be, without external stimuli, and, by the same token, what would life be for the most imperfect animals without this cause, that is to say, without the heat and electricity of the environmental surroundings.

Undoubtedly, through a sequence of this sort employed by nature, first with a feeble energy in the polyps and later with greater developments in the radiates, the radiating form was acquired. For the subtle ambient fluids, by penetrating through the alimentary canal and expanding, must have, by means of a constantly renewed expulsion from the centre towards all the points on the circumference have given rise to this radiating shape in the parts.

This is the reason why, in the radiates, the intestinal canal, although still very imperfect (because very frequently it only has one single opening) is nevertheless complicated with numerous radiating vasculiform and often branching appendixes.

No doubt, for the same reason in the soft radiates, like the jelly fish, and so on, we see a constant isochronous movement, a movement which very probably results from successive irregular movements in the masses of subtle fluids which penetrate into the interior of these animals and also the movements of these same fluids which escape after being spread throughout every part.

We should not say that the isochronous movements in the soft radiates are the consequences of their respiration. For after the vertebrate animals, nature does not manifest in any other animal alternating and measured movements of inhaling and exhaling. Whatever the respiration of radiates may be, it is extremely slow and goes on without perceptible movements.


Animals with a sub-gelatinous and regenerating body, without any special organs, other than an alimentary canal with a single opening; a terminal mouth, together with radiating tentacles or a ciliate and rotating organ.

When we reach the polyps, we have arrived at the penultimate step of the animal scale, that is to say, at the penultimate class which it has been necessary to establish among the animals.

Here, the imperfection and the simplicity in the organic structure are very evident, so that the animals at this point have hardly any more faculties and for a long time we had doubts about their animals nature.

These are gemmiparous animals, with a homogeneous body, usually gelatinous, very regenerative in their parts, not manifesting a radiating shape (something nature began in them) except in their radiating tentacles around their mouths. They have no other special organ except an intestinal canal with a single opening and, consequently, incomplete.

We can say that polyps are much more imperfect animals than those which are part of the preceding classes. For we do not find in them either brain, longitudinal marrow, nerves, special organs for respiration, vessels for circulation of fluids, nor ovaries for reproduction. The substance of their bodies is, in some way, homogenous, and made up of a gelatinous and irritable cellular tissue, in which fluids move slowly. Finally, all their internal organs are reduced to an imperfect alimentary canal, rarely folded back on itself or furnished with appendixes, looking in general like only a long sack, always with only one single opening serving simultaneously as a mouth and anus.

We can find no firm reason for claiming that in the animals in question, where we do not find a nervous system, a respiratory organ, muscles, and so on, these infinitely reduced organs nonetheless exist, but that they are spread out and dissolved in the general corporeal mass and equally distributed in all its molecules, instead of being collected together in particular places and that therefore, every point in their bodies could experience all sorts of sensations, muscular movement, will power, ideas, and thought. That would be a totally gratuitous assumption, without basis and probability. Besides, with such an assumption, one could say that the hydra has in all the points of its body, all the organs of the most perfect animals, and as a result, that each point in the body of this polyp sees, hears, distinguishes odours, notices tastes, and so on, and in addition that it has ideas, forms judgments, and thinks, in a word, that it reasons. Each molecule of the body of the hydra or of every other polyp would be by itself a perfect animal, and the hydra itself would be an even more perfect animal than human beings, because each of its molecules would be of equivalent value, in the complement of its organic structure and faculties, to a complete individual of the human race.

There is no reason to refuse to extend the same reasoning to the monad, the most imperfect of known animals and then to stop the application of this reasoning to the plants themselves, which also enjoy life. Then one would attribute to each molecule of a plant all the faculties which I have just cited, but held within the limits relative to the nature of the living body of which that molecule is a part.

That is certainly not the point to which the results of the study of nature lead. This study teaches us, by contrast, that everywhere where an organ ceases to exist, the faculties which depend upon it also cease. Every animal which has no eyes or in which the eyes have been destroyed sees nothing. And although in the last analysis the different senses derive their source from touch, which is only variously modified in each of them, every animal which lacks nerves, the special organ of feeling, will not be able to experience any sort of sensation. For it does not have the intimate sense of its existence, it does not have the chamber to which sensation must be conveyed, and consequently it would not be able to feel.

Thus, the sense of touch, the basis of the other senses, which is spread out into almost all parts of the bodies of those animals with nerves, does not exist any more in those which, like the polyps, lack nerves. In the latter, the parts are only simply irritable, and are so to a very high degree. But they have no feeling, and as a result, no type of sensation. In fact, in order for a sensation to take place, there must first be an organ to receive it (nerves) and then there must be some chamber or other (a brain or a longitudinal marrow with ganglia) where this sensation can be carried.

A sensation is always the consequence of a received impression which is carried immediately to an interior chamber where this sensation is formed. If you interrupt the communication between the organ which receives the impression and the chamber where the sensation is formed, all feeling immediately ceases in this place. One will never be able to contradict this principle.

No polyp can really be oviparous. For no polyp has a special organ for reproduction. Besides, to prepare real eggs, it is not only necessary that the animal has an ovary, but in addition it must have (or another individual of its species must have) a special organ for fertilization, and no one can demonstrate that the polyps are furnished with such organs. By contrast, we understand very well the buds which several of them develop for reproducing themselves. By paying a little attention to them, we perceive that these buds themselves are only fissions more isolated from the body of the animal, fissions less simple than those which nature uses to multiply the animalcules which make up the last class of the animal kingdom.

The polyps are very irritable, move only with external stimulations foreign to them. All their movements are the necessary results of impressions received, and are generally carried out without acts of free will, because they do not know how to produce them, since they cannot have free will.

The light forces them constantly and always in the same manner to direct themselves its way, as it does with respect to branches and leaves or the flowers of plants, although more slowly. No polyp moves after its prey or seeks for it with its tentacles. But when some foreign body touches these same tentacles, they seize it, take it into the mouth, and the polyp swallows it without making any distinction about its appropriate nature or about its utility. It digests it and feeds on it, if this body is capable of being digested. It rejects it entirely, if it is preserved intact for some time in the alimentary canal. Finally it returns any remains which it cannot break down any more. But in all that, the action is a necessary one, without ever the possibility of a choice which permits the action to vary.

As to the distinction between the polyps and the radiates, it is very large and marked. We do not find in the interior of the polyps any distinct part having a radiating arrangement. Their tentacles alone have this arrangement, that is to say, the same as the arrangement of the arms of the cephalid mollusks, which we will surely not mix up with the radiates. Moreover, the polyps have a terminal superior mouth, while the mouth of the radiates is arranged differently.

It is not at all convenient to call polyps zoophytes, which is to say, animal-plants, because they are uniquely and completely animals, in that they have faculties generally not found in plants, that of being truly irritable and, generally, that of digesting. Finally their essential nature does not tend toward that of plants.

The only interrelationships between polyps and plants are as follows: (1) the rather similar simplification in their organic structure, (2) the faculty which many polyps possess of adhering to one another, to communicating together by their alimentary canal, and to form composite animals, (3) finally, the exterior shape of the masses which these combined polyps form. This shape for a long time made people take these masses for real plants, because often they are branched out in almost the same way.

Whether the polyps have a single or several mouths, it is always the case, with respect to them, that they lead to a single alimentary canal and, consequently, to one digestive organ, which all the plants lack.

If the degradation in the organic structure which we have noticed in all the classes from the mammals on is anywhere evident, it is surely among the polyps, whose organic structure is reduced to an extreme simplicity.


Infinitely small animals, with gelatinous transparent bodies, homogenous and very contractile; having no specially distinct interior organ, but often with oviform gemmules, and displaying on the outside no radiating tentacles nor rotary organs.

Finally, we have arrived here at the last class in the animal kingdom, the one which makes up the most imperfect animals in all respects, that is, those whose organic structure is the simplest, who possess the fewest faculties, and who all seem, in truth, to be only sketches of animal nature.

Up to the present, I have combined these small animals in the class of polyps, of which they make up the last order under the name amorphous polyps, having no constant shape which is unique to all. But I recognized the necessity of separating them to create a special class. This does nothing to change the rank which I had assigned to them. All that results from this change is limited to a line of separation which the greatest simplification in their organic structure and their lack of radiating tentacles and rotary organs seem to require.

Since the organic structure of infusorians becomes increasingly simple, according to the genera which make up the class, the last of these genera present to us, in some way, the limit of animal nature. At least, the animals display the limit which we can reach. Particularly in the animals of the second order of this class, it is certain that all traces of the intestinal canal and mouth have disappeared entirely, that there are no organs at all, in a word, that they no longer carry out digestion.

The infusorians are only very small gelatinous bodies, transparent, contractile and homogenous, made up of cell tissue almost without any consistency, yet nevertheless irritable at all points. These small bodies, which appear only animated or moving points, nourish themselves by absorption and a constant water intake. Undoubtedly they are animated through the influence of subtle ambient fluids, like heat and electricity, which simulate in them the movements which make up their lives.

In considering such animals, if we were still to assume that they possessed all the organs which we know about in other animals but that these organs are based in all the points of their bodies, how vain would such an assumption be!

In fact, the extremely weak (almost nonexistent) consistency of the parts of these small gelatinous bodies indicates that such organs must not exist, because it would be impossible for them to carry out their functions. It is clear enough that, in order for any organs to have the power to react upon fluids and to exert their appropriate functions, their parts must have the consistency and the tenacity which can give them force; now, this is precisely what cannot be assumed in connection with the frail animals under discussion.

Only among the animals of this class does nature appear to create spontaneous or direct generations, which she renews continually whenever circumstances are favorable for that. We will try to show that by spontaneous generations nature has acquired a way to create indirectly, after the passage of an enormous time, all the other races of animals which we know about.

What justifies our thinking that the infusorians or the majority of these animals owe their existence purely to spontaneous generations is that these frail animals all perish in the low temperatures brought on by the bad weather seasons. For surely we will not assume that such delicate bodies can leave any bud with sufficient consistency to preserve itself and to reproduce in warm seasons.

We find the infusorians in stagnant waters, in the infusions of plant or animal substances, and even in the reproductive liquid of the most improved animals. We find them exactly alike in all parts of the world, but only under circumstances where they could be created.

Thus, by considering successively the different systems of organic structures in animals, from the most complex right to the simplest, we have witnessed the degradation in that structure start in the very class which includes the most improved animals, then later progressively advance from class to class, although with anomalies produced by various sorts of circumstances, and finally, end in the infusorians. These last are the most imperfect animals, the simplest in organic structure, and those in which the degradation which we have followed has reached its limit, by reducing the animal's organic structure so that it consists of a simple body, homogenous, gelatinous, almost without consistency, without special organs, and uniquely formed from a very delicate cellular tissue, hardly sketched out, which appears to be brought to life by subtle ambient fluids which penetrate it and move out again constantly.

We have seen the successive process by which each particular organ, even the most essential, gets degraded little by little and finally goes away and disappears entirely long before reaching the other end of the order which we are following. And we have noticed that it is principally in the invertebrate animals that we see particular organs done away with.

To be sure, even before leaving the vertebrate division, we already perceive great changes in the improved condition of the organs, and some of them, like the urinary bladder, diaphragm, vocal organ, eyelids, and so on, even disappear totally. In fact, the lung, the most improved organ for respiration, begins to be degraded in the reptiles and ceases to exist in the fish, never to reappear in any invertebrate animal.

Finally, the skeleton, the basis for the four extremities or limbs which the majority of vertebrate animals possess, begins to deteriorate, mainly in the reptiles, and is completely finished in the fish.

But it is in the division of invertebrate animals that we see the disappearance of the heart, brain, gills, conglomerate glands, the vessels appropriate for circulation, the organs for hearing, sight, sexual generation and even of feeling, as well as those for movement.

As I have already stated, it would be vain for us to seek out in a polyp, for example in a hydra, or in the majority of animals of this class, the least vestiges of nerves (organs of feeling) or muscles (organs of movement). Only irritability, with which every polyp is very clearly endowed replaces in it both the faculty of feeling which it cannot possess (because it does have any organ essential for that) and the faculty of voluntary movement (because all voluntary action is an act of the organ of intelligence and this animal totally lacks any organ like that). All its movements are the necessary results of impressions received in its irritable parts, exterior stimuli, and are carried out without any possibility of choice.

Put a hydra in a glass of water and place this glass in a room which does not receive light except through a window (hence, from a single side). When the hydra is established on a point of the sides of the glass, turn the glass in such a manner that the daylight strikes another point opposite to the one where the animal is located. You will always see the hydra go in a slow movement to take up a position in the place where the light strikes and stay there for as long as you do not change the point. In doing this, the hydra follows what we see in the parts of plants which arrange themselves, without any act of will power, towards the quarter from which light comes.

Undoubtedly, wherever a particular organ no longer exists, the faculty which it gives rise to also ceases to exist. But, in addition, we see clearly that to the extent that an organ deteriorates and gets smaller, the faculty deriving from it becomes proportionally more obscure and imperfect. Thus, as we descend from the most complex toward the simplest animals, the insects are the last animals in which one finds eyes. But we have every reason to believe that they see very obscurely and make little use of their sight.

Thus, in moving through the animal chain from the most perfect to the most imperfect and by considering successively the different systems of organic structure which we distinguish in the extent of this chain, we see that the degradation in organic structure and of each organ (right to their total disappearance) is a reliable fact whose reality we have just confirmed.

This degradation is manifested even in the nature and consistency of the essential fluids and the flesh of animals. For the flesh and blood of mammals and birds are the most complex and vital materials that one can obtain from animals' soft parts. In addition, after the fish these materials progressively deteriorate to the point where in the soft radiates, polyps, and above all in the infusorians, the essential fluid has nothing any more except the consistency and colour of water, and the flesh of these animals displays only a gelatinous material, hardly animalised. The soup which one might make out of such flesh would undoubtedly prove scarcely nourishing and strengthening for the person to tried it out.

Whether one acknowledges or not these interesting truths, nonetheless, those who observe facts carefully, overcome the widespread general prejudices, consult natural phenomena, and study nature's laws and constant march will always be led to them.

Now we are going to move on to consider something else, and we will try to prove that the environmental circumstances exercise a large influence on the actions of animals and that, as a result of this influence, the increased and sustained use of an organ or its lack of use are causes modifying the organic structure and shape of animals and giving rise to the anomalies which we observe in the structural progress within animal organization.



Notes to Chapter Six

(1) If birds have their lungs pierced and their hair changed into feather as a consequence of their habit of ascending into the air, people will ask me why the bats do not also have feathers and pierced lungs. I will reply that it seems to me probable that the bats have a systematic organic structure more perfect than that of the birds, and as a result a complete diaphragm which limits the enlargement of their lungs. Thus they were not able successfully to pierce their lungs nor to puff themselves up with air sufficiently so that the influence of this fluid, with effort reaching the skin, gives the horny material of the hair the faculty of branching out into feathers. In fact, in the birds, the air, once introduced right into the bulb of the hairs, changes their base into a tube and forces these very hairs to divide themselves up into feathers. This could not have happened with the bat, where the air does not penetrate beyond the lung. [Back to Text]

(2) The Anatifa, Balanus, Coronula and Tubicinella. [Back to Text]

(3) "It is above all in the spiders that the heart is easy to observe. We see it beat through the skin of the abdomen in the non-hairy species. In lifting up this skin, we see a hollow organ, oblong, pointed at two ends, the anterior end pointing towards the thorax, on the sides of which two or three pairs of vessels can be seen leaving." Cuvier, Anatom. Comp. Vol. IV, p. 419. [Back to Text]



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