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For the progress of Philosophical Zoology and for the goal which we have in mind, it is necessary to consider the present state of the distribution and classification of animals, to examine how we have arrived at it, to recognize what the principles are to which we had to conform to establish this universal distribution, and finally to study what remains to accomplish to give this distribution the most appropriate character so as to make it depict the order of nature itself.
But to derive some benefit from all these matters, we must first determine the essential purpose of the distribution of animals and of their classification, because these two purposes have very different natures.
The purpose of a general distribution of animals is not only to obtain a convenient list to consult, but above all to have in this list an order which represents, as much as possible, the very order of nature, that is to say, the order which nature has very clearly characterized by the relationships which she has established between the animals.
The aim of a classification of animals, by contrast, is to furnish, with the help of lines of separation traced in various places in the general series of creatures, points where our imagination can stop so that we can more easily recognize each race which has already been observed, grasp the interconnections with the other known animals, and place in each group the new species which we succeed in discovering. This method compensates our weaknesses, makes our studies and our understanding easier, and its use is indispensably necessary to us. But I have already shown that it is a product of art and that, despite any appearance to the contrary, it has no real connection with nature.
The proper determination of the interconnections between things will always invariably establish in our general distributions, first, the place of the large groups or the primary divisions and, second, that of the groups subordinate to the first and, finally, that of the species or the particular races which have been observed. Now, there you see the inestimable advantage for science of the knowledge of the interrelationships: since these interrelationships are the work of nature itself, no naturalist would ever have the power or, no doubt, the desire to change the results of a known interrelationship. The general distribution thus will become more and more perfect and compelling to the extent that our understanding of the interconnections advances with respect to the things which make up a kingdom.
The case is different with classification, that is to say, with the different lines of separation which we are concerned to trace from place to place in the general distribution, whether of animals or plants. In truth, as long as there will be spaces to fill in our distributions, because many animals and plants have not yet been observed, we will always find with these lines of separation that they seem to us created by nature itself. But this illusion will vanish to the extent that we continue to observe. And already have we not seen a sufficiently large number of them disappear, at least in the smallest groups, through the numerous discoveries of naturalists in roughly the last fifty years?
Thus, apart from the lines of separation which are the result of gaps which we have to fill, those which we will always be forced to create will be arbitrary and hence changeable, as long as naturalists do not adopt some conventional principle to regulate themselves when they draw such lines.
In the animal kingdom, we should consider as a conventional regulatory principle the fact that every class should be understood to consist of animals characterized by a particular structural system. The strict application of this principle is easy enough and presents only moderate inconveniences.
In fact, although nature does not move abruptly from one structural system to another, it is possible to establish some limits between each system, so that throughout nature there is only a small number of animals placed near these limits and in a position to raise some doubts about their true class.
The other lines of separation which sub-divide the classes are, in general, more difficult to fix, because they rest on less important characteristics and, for this reason, are more arbitrary.
Before examining the present state of the classification of animals, let me attempt to show that the distribution of living beings must form a series, at least with the large groups, and not a ramified network.
As human beings are condemned to go through all possible mistakes before recognizing one truth when they examine the facts which are relevant to it, people have denied that the productions of nature in each kingdom of living creatures were really in the position of being able to form a true series which follows a consideration of their interrelationships, and we have not wished to recognize any scale in the general arrangement, whether of animals or plants.
Thus, since naturalists have noticed that many species, certain genres, and even some families appear in a sort of isolation so far as their characteristics are concerned, several of them have imagined that the living creatures in one kingdom or the other, which according to their natural interconnections are closely related to or very distant from each other, are distributed like the different points of a geographical map or a globe. They consider the small and very pronounced series which have been called natural families as organized in the form of a network. This idea, which has appeared sublime to some moderns, is clearly a mistake. And it will undoubtedly disappear as soon as we have a more profound and more universal understanding of organic structure, above all when we distinguish what belongs to the influence of habitats and acquired habits from what is the result of more or less advanced progress in the composition or the perfecting of organic structures.
In the meantime, I am going to show that nature gives rise, with the help of a great deal of time, to the existence of all animals and plants and has really established in each of these kingdoms a true ladder with respect to the growing complexity in the organic structure of these living beings, but that this ladder, which it is our concern to recognize in dealing with these things according to their natural interrelationships, only offers comprehensible gradations in the main large groups of the universal series and not in species, nor even in the genres. The reason for this point stems from the fact that the extreme diversity of circumstances in which the different races of animals and plants find themselves is not related to the increasing complexity in the organic structure among them, as I will show, and that gives rise in their shapes and exterior characteristics to anomalies or types of leaps which the increasing complexity in the organic structure could not have brought about by itself.
It is therefore a matter of proving that the series which makes up the animal ladder consists essentially in the distribution of the principle groups which make it up and not in the distribution of species, nor even in that of the genera.
The series which I am going to talk about could not therefore be established except in the placing of the large groups, because these groups which make up the classes and the large families each consist of beings in which the organic structure depends upon some particular system of essential organs.
Thus each distinct large group has its own particular system of essential organs. And it is these particular systems which are going to deteriorate from the one which shows the greatest complexity right to the one which is the simplest. But each organ considered in isolation does not follow such a regular path in its deterioration. To the extent that the organ has less importance and is more susceptible to being modified by circumstances, it follows such a path even less.
In fact, the organs with little importance or inessential to life are not always similar to each other in their perfection or degradation, so that if we follow all the species of a single class, we will see that a particular organ in a particular species enjoys the highest degree of perfection, while some other organ which, in this same species, is much impoverished or very imperfect, is found very perfected in some other species.
These irregular variations in the perfection and the degradation of non-essential organs come up in those organs more subject than others to the influence of external circumstances. This influence brings with it similar variations in the shape and in the nature of the most external parts and gives rise to such a large and strangely organized diversity among species, that instead of being able to arrange them, like the large groups, in a unique series, simple and linear, in the form of a regularly graduated scale, these same species often form around the large groups of which they are a part, lateral branches, whose extremities display truly isolated points.
To modify each interior system of organic structure requires a more influential combination of circumstances and a much longer period of time than to modify and change the external organs.
Nevertheless, I notice that when circumstance demand, nature goes from one system to another, without making a jump, provided that the systems are closely related. In fact, it is by this faculty that nature has managed to form all systems successively, going from the most simple to the most complex.
It is so true that nature has this faculty, that it moves from one system to another, not only in two different families when they are related by their interconnections, but even in a single individual.
The system of organic structure which include as organs of respiration real lungs are more closely related to systems which include gills than those which require tracheae. Thus, not only does nature move from gills to lungs in the neighboring classes and families, as a consideration of fish and reptiles shows, but it moves there even during the life of individuals themselves, who enjoy successively both systems. We know that frogs, in the imperfect tadpole state, breathe by gills; whereas, in their more perfect condition as frogs they breathe by lungs. We do not see anywhere nature passing from a system of tracheae to a pulmonary system.
Thus, it is true to say that there exists for each kingdom of living beings a unique and graduated series in the arrangement of the large groups, in conformity with the increasingly complexity in the organic structure and with the arrangement of things according to a consideration of the interrelationships and that this series, whether in the animal or plant kingdom, must offer at its front extremity the simplest and the least organized living creatures and finish with the most perfect in structure and faculties.
Such appears to be the true order of nature, and such is effectively what the most attentive observation and a sustained study of all the features which characterize its progress clearly present to us.
Since the time when, in our distributions of the production of nature, we have felt it necessary to concern ourselves with the interrelationships, we are no longer masters at arranging the universal series as we please. The knowledge which we increasingly acquire of nature's progress, to the extent that we study the close or distant interrelationships which it has established, whether between objects or between their different groups, carries us along with it and forces us to conform to nature's order.
The first result obtained from the use of interrelationships in the placement of the large groups to form a general distributions is that the two extreme ends in the order must display the most dissimilar beings, because they are effectively the most distant so far as such interrelationships are concerned and, as a result, so far as organic structure is concerned. It follows from this that if one of the extremities of the order shows living beings developed in the most perfect way, those in which the organic structure is the most complex, the other extremity of the same order must necessarily show the most imperfect living creatures, that is to say, those in which the organic structure is the simplest.
In the general disposition of the known plants, according to the natural method, that is to say, according to the consideration of the interrelationships, as yet we understand reliably only one of the extremities of the order: we know that the cryptogram must be located at this extremity. If the other extremity is not established with the same certainty, that stems from the fact that our knowledge of the organic structure of plants is much less advanced than what we understand about a large number of known animals. Consequently, so far as plants are concerned, we do not yet have a certain guide to determine the interrelationships between the large groups, of the sort that we have for recognizing those which exist between the genera and which form the families.
We do not encounter the same difficulty with the animals. The two extremities of their general series are determined in a definite way. For as long as we attach importance to the natural method and, as a result, to the consideration of the interrelationships, the mammals will necessarily occupy one of the extremities of the order, whereas, the infusorians will be placed at the other extremity.
Therefore, there is for animals, as well as for plants, an order which belongs to nature and which, like the things which this order brings into existence, results from the methods which nature has received from the Supreme Author of all things. It is nothing other than the universal immutable order which this Sublime Author has created in everything, together with the collection of general and particular laws to which this order is subject. By these means, which nature continues to use, it has given and perpetually is giving life to its productions. It varies them and renews them without ceasing, and in this way maintains the entire order which comes from these means.
We are going to see that this natural order which we were concerned successfully to recognize in each kingdom of living beings and of which we already possess various sections in the well known families and in our best genera, is, so far as the animal kingdom is concerned, now determined in its entirely in a manner which leaves no room for anything arbitrary.
But the large number of the various animals which we have succeeded in understanding and the numerous insights which comparative anatomy has provided about their organic structure now give us the means of determining, in a definitive manner, the general distribution of all the known animals and of assigning a confirmed rank for the main divisions which we can establish in the series which they form.
That is what it is important to recognize and what will be truly difficult to dispute.
Let us now move on to an examination of the present state of the general distribution of animals and of their classification.
Since the purpose and the principles, whether of the general distribution of living beings or of their classification, have not been noticed when we concerned ourselves with these matters, the works of naturalists suffered for a long time from this imperfection of our ideas. The natural sciences were like all the others which we busied ourselves with for a long time before thinking about the principles which must form the basis of the science and govern the work which goes on in it.
Instead of subjecting the classification necessary in each kingdom of living creatures to a distribution which nothing should interfere with, we thought only of classifying objects conveniently and in this way of subjecting their distribution to arbitrariness.
For example, since the connections between the large groups were very difficult to grasp among the plants, for a long time in botany we used artificial systems. They made convenient classifications easy to create, based on arbitrary principles. And each author made up new ones according to his fancy. Thus the distribution we need to establish among the plants, the one which, in a word, belongs to the natural method, was then always sacrificed. Only since we have understood the importance of the parts concerned with the fruit, and above all the preeminence which certain of those parts must have over the others, has the general distribution of plants started to progress towards perfection.
Since the case is not the same so far as animals are concerned, the general connections which characterize the large groups are, among themselves, a great deal easier to perceive. Also several of these groups have been recognized since the time when we first began to cultivate natural history.
In fact, Aristotle, at the start, divided the animals in two main groups or, according to him, two classes, as follows:
Animals with Blood
Viviparous quadrupeds
Oviparous quadrupeds
Fish
Birds
Animals without Blood
Mollusks
Crustaceans
Testaceans
Insects
This primary division of animals into two large groups was good enough, but the characteristic used by Aristotle in drawing it up was poor. This philosopher gave the name blood to the principal fluid in animals which is red in colour. Assuming that since all the animals which belong to his second class possessed only white or off white fluids, he therefore considered that they lacked blood.
Such was apparently the first sketch of a classification of animals. It is, at least, the oldest we know about. But this classification also gives the first example of a distribution in an sense reversed from the natural order, since we find in it a progression, although a very imperfect one, from the most complex to the simplest.
Since this time, people have generally followed this false direction with regard to the distribution of animals. And this has clearly held back our knowledge concerning the nature's march.
Modern naturalists believed that they were perfecting Aristotle's distinction when they gave to the animals of his first division the name red-blooded animals and to those of the second division the name white-blooded animals. We are sufficiently aware now how much this characteristic is defective, since there are invertebrate animals with red blood (many annelids).
In my view, the fluids essential to animals cease to merit the name blood as soon as they no longer circulate in arterial and venous vessels. These fluids are then so degraded, so lacking in complexity or so imperfect in the combination of their principles, that we were wrong to link their nature to that of fluids which undergo a true circulation. Besides, attributing blood to a radiate or to a polyp means as much as attributing blood to a plant.
To remove all ambiguity or the use of any hypothetical consideration, in my first course of study which I carried out in the Museum, in the spring of 1794 (Year Two of the Republic), I divided all the known animals into two perfectly distinct groups:
Animals with backbones
Animals without backbones.
I drew my students' attention to the fact that the vertebral column indicates, in the animals which have it, the possession of a skeleton more or less perfect and of a structural plan relative to it; whereas, the lack of a vertebral column in the other animals not only clearly distinguishes them from the first ones, but announces that the structural plans on which they have been developed were all very different from those of the vertebrate animals.
From Aristotle up to Linnaeus, nothing very noteworthy has appeared concerning the general distribution of animals. But in the last century, some naturalists of exceeding merit made a large number of particular observations concerning animals, mainly on a number of animals without vertebrae. Some revealed their anatomy with more or less detail; others provided an exact and detailed history of the changes in and the habits of a large number of these animals. As a result of their valuable observations, we have come to understand many facts of the highest importance.
Finally Linnaeus, a man of a superior genius and one of the greatest known naturalists, after having collected the facts and taught us to use great accuracy in the determination of characteristics for all the orders, gave us the following distribution for the animals
He distributed the known animals in six classes, on the basis of three stages or characteristics of their organic structure.
|
Classes |
First Stage |
|
I. Mammals
|
A heart with two ventricles, red blood and warm |
|
Second Stage | |
|
III. Amphibians (the Reptiles)
|
Heart with one ventricle, red blood, and cold |
|
Third Stage | |
|
V. Insects
|
A cold serum (in place of blood) |
Except for the inversion which this distribution manifests, like all the others, the four first divisions which it presents are now definitely fixed and will always be accepted by zoologists insofar as their position in the general series. And we see that it is the illustrious Swedish naturalist whom we have primarily to thank for this.
The case is not the same with the last two divisions of the distribution under discussion. They are poor and very badly arranged. Since they include the greatest number of known animals and those with the most diverse characteristics, they should be more numerous. It is therefore necessary to reformulate them and to substitute others for them.
Linnaeus, as we see, and the naturalists who followed him, gave so little attention to the need to multiply the divisions among the animals with a cold serum in place of blood (the animals without vertebrae) and those in which the characteristics present such a great variety, that they distinguished these numerous animals in only two classes, as follows: in insects and worms. As a result, everything which was not considered an insect, or alternatively, all animals without vertebrae which did not have articulated members were, without exception, included in the class of worms. They placed the class insect after the fish, and the class worms after the insects. The worms therefore formed, according to this distribution of Linnaeus, the last class of the animal kingdom.
These two classes are still found laid out, following this order, in all the editions of Systema naturae published after Linnaeus. And although the essential problem of this distribution, so far as the natural order of animals is concerned, is evident and we cannot deny that Linnaeus' class worms is a sort of chaos in which things very different are found united, the authority of this scholar was so weighty among naturalists, that no one dared to change this monstrous class worms.
Intending to institute some useful reforms in this matter, I presented in my first course of study the following distribution for the animals without vertebrae, which I divided, not into two classes, but into five in the order given below:
Distribution of Animals Without Vertebrae as Laid Out in My First Course of Study
Mollusks;
Insects
Worms
Echinoderms;
Polyps
These classes were made up then of some of the orders which Bruguiere had presented in his distribution of worms, an arrangement I did not adopt, and from the class insects, such as Linnaeus had described it.
However, towards the middle of Year Three of the revolution (1795), when the arrival of Cuvier in Paris directed the attention of zoologists to the organic structure of animals. I saw, with much satisfaction, the decisive proofs which he provided for the preeminence which must be given to the mollusks over the insects, so far as concerns the rank which these animals must occupy in the general series, something which I had already carried out in my classes but which had not been viewed favorably on the part of naturalists of that capital city.
The change which I had made in this matter, from a sense of the inconvenience in Linnaeus' distribution which people followed, Cuvier endorsed perfectly through his explanation of the most reliable facts, among which several, in truth, were already known, but which had not yet attracted our attention in Paris.
Profiting then from the illumination which this scholar, since his arrival, has shed on all parts of zoology, and especially on the animals without backbones, which he called animals with white blood, I added successively new classes to my distribution. I was the first to institute them. But, as we are going to see, the classes of mine which were adopted were only accepted slowly.
No doubt, what interests authors is a matter of total indifference to science and also to those who study the subject. Nevertheless, there is a practical value in knowing the history of changes which the classification of animals has undergone in the past fifteen years. Here are those which I have effected.
To begin with, I changed the denomination of my class of echinoderms into that of Radiata, in order to unite in it the jelly fish and the genera which are related to them. This class, in spite of its utility and the necessity for it on account of the characteristics of these animals has not yet been adopted by the naturalists.
In my course for Year Seven (1799), I established the class crustaceans. At that time Cuvier in his Table of Animals, page 451, still included the crustaceans among the insects, and although this class is essentially distinct from the insects, nonetheless, the naturalists consented to adopt it only six or seven years afterwards.
The following year, that is to say, in my course for Year Eight (1800), I presented the arachnids as a special class, easy and necessary to distinguish. The nature of its characteristics were from that time on a certain indication of an organic structure peculiar to these animals, for it is impossible that an organic structure perfectly suitable to the insects, who all undergo metamorphoses, reproduce themselves only once in the course of their lives, and have only two antennae, two faceted eyes, and six articulated limbs, could give rise to animals which never undergo metamorphosis and which, in addition, present different characteristics which distinguish them from the insects. A part of this truth has since been confirmed by observation. However, this class arachnids has not yet been admitted in any work other than mine own.
Once Cuvier discovered the existence of arterial and venous vessels in different animals which people confused under the name worms with other animals very differently structured, I immediately used the implication of this new fact to perfect my classification. In my course for the Year Ten (1802), I established the class annelids, a class which I placed after mollusks and before crustaceans, something required by their acknowledged organic structure.
In giving a particular name to this new class, I was able to keep the ancient name worms for the animals which had always carried it and whose organic structure obliged me to distance them from annelids. Thus, I continued to place worms after insects and to distinguish them from radiata and polyps, with which people will never be authorized to unite them again.
My class annelids published in my course and in my Researches into Living Beings (p. 24) was around for several years without being accepted by naturalists. Nevertheless, for about the past two years, people are starting to recognize this class. But since people are of the opinion that they should change its name and to bring in the name worms for it, they do not know what to do with the creatures properly called worms, which do not have nerves, nor a system of circulation, and in this quandary, they are reuniting them with the class polyps, even though they are very different from them in their organic structure.
These examples of perfection first established in parts of a classification and later destroyed by others and later reestablished by the necessity and the pressure of things are not uncommon in the natural sciences.
In fact, Linnaeus combined several plant genera which Tournefort had previously separated, as one sees in his general polygonum, mimosa, justicia, convallaria, and plenty of others. And now the botanists are reestablishing the genera which Linnaeus had destroyed.
Finally, last year (in my course for 1807), I established among the animals without vertebrae a new tenth class, the infusorians, because after a sufficient examination of the known characteristics of these imperfect animals, I was convinced that I had been wrong to include them among the polyps.
Thus, in continuing to collect the facts gained through observation and through the rapid progress in comparative anatomy, I instituted successively the different classes which now make up my distribution of the animals without vertebrae. These classes, ten in number, are arranged from the most complex to the simplest, as is the custom, as follows:
I will reveal in dealing with each of these classes that they constitute the necessary divisions, because they are based on a consideration of the organic structure and that, although it may be or indeed must be the case that we find in the vicinity of the limits to the classes some races, in one way or another, half way or intermediate between two classes, these divisions offer everything which art can produce which is most helpful in this sort of endeavour. Thus, as long as our main concern is an interest in science, people will not be able to do without acknowledging them.
It will be noticed that, by adding to these ten classes which divide the animals without backbones the four classes recognized and fixed by Linnaeus among the animals with vertebrae, we will have for the classification of all known animals the fourteen following classes, which I am going once more to present in an order opposite to the natural order.
Vertebrate Animals
Mammals
Birds
Reptiles
Fish
Invertebrate Animals
Mollusks
Cirrhipeds
Annelids
Crustaceans
Arachnids
Insects
Worms
Radiata
Polyps
Infusorians
Such is the present state of the general distribution of animals, and such is the distribution of classes which have been established among them
It will be a matter now of examining a very important questions which appears never to have been explored or discussed. However, the solution to it is essential. Here it is.
Since all the classes which are part of the animal kingdom form necessarily a series of large groups according to the growing complexity or diminishing complexity in their organic structure, must we, in the arrangement of this series, proceed from the most complex to the simplest or from the simplest to the most complex?
We will try to give the solution to this question in Chapter VIII which brings this part of the book to a conclusion. But before that we must examine a very remarkable fact, most worthy of our attention, which can lead us to notice the march nature follows in giving its various productions the existence which they enjoy. I am going to talk about the remarkable degradation which is found in all organic structure, if one moves through the natural series of animals, beginning with the most perfect or the most complex and moving towards the simplest and the most imperfect.
Although this degradation is not nor can be finely demarcated, as I will reveal, it exists in the main groups so evidently and with a sustained consistency, even in the variations in the path, that it depends, no doubt, on some general law which it is important for us to discover and, consequently, to seek out.
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