Origin of Species
'But with regard to the material world, we can at least go so far as this--we can perceive that events are brought about not by insulated interpositions of Divine power, exerted in each particular case, but by the establishment of general laws.' W. Whewell: Bridgewater Treatise.
'To conclude, therefore, let no man out of a weak conceit of sobriety, or an ill-applied moderation, think or maintain, that a man can search too far or be too well studied in the book of God's word, or in the book of God's works; divinity or philosophy; but rather let men endeavour an endless progress or proficience in both.' Bacon: Advancement of Learning.
Down, Bromley, Kent, October 1st, 1859.
Causes of Variability--Effects of Habit Correlation of Growth--Inheritance--Character of Domestic Varieties--Difficulty of distinguishing between Varieties and Species--Origin of Domestic Varieties from one or more Species--Domestic Pigeons, their Differences and Origin--Principle of Selection anciently followed, its Effects--Methodical and Unconscious Selection--Unknown Origin of our Domestic Productions--Circumstances favourable to Man's power of Selection.
Variability--Individual Differences--Doubtful species--Wide ranging, much diffused, and common species vary most--Species of the larger genera in any country vary more than the species of the smaller genera--Many of the species of the larger genera resemble varieties in being very closely, but unequally, related to each other, and in having restricted ranges.
Bears on natural selection--The term used in a wide sense - Geometrical powers of increase--Rapid increase of naturalised animals and plants--Nature of the checks to increase--Competition universal--Effects of climate--Protection from the number of individuals--Complex relations of all animals and plants throughout nature--Struggle for life most severe between individuals and varieties of the same species; often severe between species of the same genus--The relation of organism to organism the most important of all relations.
Natural Selection--its power compared with man's selection--its power on characters of trifling importance--its power at all ages and on both sexes--Sexual Selection--On the generality of intercrosses between individuals of the same species--Circumstances favourable and unfavourable to Natural Selection, namely, intercrossing, isolation, number of individuals--Slow action--Extinction caused by Natural Selection--Divergence of Character, related to the diversity of inhabitants of any small area, and to naturalisation--Action of Natural Selection, through Divergence of Character and Extinction, on the descendants from a common parent--Explains the Grouping of all organic beings.
Effects of external conditions--Use and disuse, combined with natural selection; organs of flight and of vision--Acclimatisation - Correlation of growth--Compensation and economy of growth--False correlations--Multiple, rudimentary, and lowly organised structures variable - Parts developed in an unusual manner are highly variable: specific characters more variable than generic: secondary sexual characters variable--Species of the same genus vary in an analogous manner--Reversions to long-lost characters--Summary.
Difficulties on the theory of descent with modification--Transitions--Absence or rarity of transitional varieties--Transitions in habits of life--Diversified habits in the same species--Species with habits widely different from those of their allies--Organs of extreme perfection--Means of transition--Cases of difficulty--Natura non facit saltum--Organs of small importance--Organs not in all cases absolutely perfect--The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural Selection.
Instincts comparable with habits, but different in their origin--Instincts graduated--Aphides and ants--Instincts variable - Domestic instincts, their origin--Natural instincts of the cuckoo, ostrich, and parasitic bees--Slave-making ants--Hive-bee, its cell-making instinct--Difficulties on the theory of the Natural Selection of instincts--Neuter or sterile insects--Summary.
Distinction between the sterility of first crosses and of hybrids--Sterility various in degree, not universal, affected by close interbreeding, removed by domestication--Laws governing the sterility of hybrids--Sterility not a special endowment, but incidental on other differences--Causes of the sterility of first crosses and of hybrids--Parallelism between the effects of changed conditions of life and crossing--Fertility of varieties when crossed and of their mongrel offspring not universal--Hybrids and mongrels compared independently of their fertility-- Summary.
On the absence of intermediate varieties at the present day--On the nature of extinct intermediate varieties; on their number--On the vast lapse of time, as inferred from the rate of deposition and of denudation--On the poorness of our palaeontological collections--On the intermittence of geological formations--On the absence of intermediate varieties in any one formation--On the sudden appearance of groups of species--On their sudden appearance in the lowest known fossiliferous strata.
On the slow and successive appearance of new species--On their different rates of change--Species once lost do not reappear--Groups of species follow the same general rules in their appearance and disappearance as do single species--On Extinction--On simultaneous changes in the forms of life throughout the world--On the affinities of extinct species to each other and to living species--On the state of development of ancient forms--On the succession of the same types within the same areas--Summary of preceding and present chapters.
Present distribution cannot be accounted for by differences in physical conditions--Importance of barriers--Affinity of the productions of the same continent--Centres of creation--Means of dispersal, by changes of climate and of the level of the land, and by occasional means - Dispersal during the Glacial period co-extensive with the world.
Distribution of fresh-water productions--On the inhabitants of oceanic islands--Absence of Batrachians and of terrestrial Mammals--On the relation of the inhabitants of islands to those of the nearest mainland--On colonisation from the nearest source with subsequent modification--Summary of the last and present chapters.
Classification, groups subordinate to groups--Natural system--Rules and difficulties in classification, explained on the theory of descent with modification--Classification of varieties--Descent always used in classification--Analogical or adaptive characters--Affinities, general, complex and radiating--Extinction separates and defines groups--Morphology, between members of the same class, between parts of the same individual--Embryology, laws of, explained by variations not supervening at an early age, and being inherited at a corresponding age - Rudimentary Organs; their origin explained--Summary.
Recapitulation of the difficulties on the theory of Natural Selection--Recapitulation of the general and special circumstances in its favour--Causes of the general belief in the immutability of species--How far the theory of natural selection may be extended--Effects of its adoption on the study of Natural history--Concluding remarks.