The Foundations of the Origin of Species - Two Essays written in 1842 and 1844
by Charles Darwin
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{474} Origin, Ed. i. p. 444, vi. p. 610.

{475} In Var. under Dom., Ed. ii. vol. I. p. 295, such eggs are said to be laid early in each season by the black Labrador duck. In the next sentence in the text the author does not distinguish the characters of the vegetable capsule from those of the ovum.

If then the two following propositions are admitted (and I think the first can hardly be doubted), viz. that variation of structure takes place at all times of life, though no doubt far less in amount and seldomer in quite mature life{476} (and then generally taking the form of disease); and secondly, that these variations tend to reappear at a corresponding period of life, which seems at least probable, then we might a priori have expected that in any selected breed the young animal would not partake in a corresponding degree the peculiarities characterising the full-grown parent; though it would in a lesser degree. For during the thousand or ten thousand selections of slight increments in the length of the limbs of individuals necessary to produce a long-limbed breed, we might expect that such increments would take place in different individuals (as we do not certainly know at what period they do take place), some earlier and some later in the embryonic state, and some during early youth; and these increments would reappear in their offspring only at corresponding periods. Hence, the entire length of limb in the new long-limbed breed would only be acquired at the latest period of life, when that one which was latest of the thousand primary increments of length supervened. Consequently, the foetus of the new breed during the earlier part of its existence would remain much less changed in the proportions of its limbs; and the earlier the period the less would the change be.

{476} This seems to me to be more strongly stated here than in the Origin, Ed. i.

Whatever may be thought of the facts on which this reasoning is grounded, it shows how the embryos and young of different species might come to remain less changed than their mature parents; and practically we find that the young of our domestic animals, though differing, differ less than their full-grown parents. Thus if we look at the young puppies{477} of the greyhound and bulldog—(the two most obviously modified of the breeds of dog)—we find their puppies at the age of six days with legs and noses (the latter measured from the eyes to the tip) of the same length; though in the proportional thicknesses and general appearance of these parts there is a great difference. So it is with cattle, though the young calves of different breeds are easily recognisable, yet they do not differ so much in their proportions as the full-grown animals. We see this clearly in the fact that it shows the highest skill to select the best forms early in life, either in horses, cattle or poultry; no one would attempt it only a few hours after birth; and it requires great discrimination to judge with accuracy even during their full youth, and the best judges are sometimes deceived. This shows that the ultimate proportions of the body are not acquired till near mature age. If I had collected sufficient facts to firmly establish the proposition that in artificially selected breeds the embryonic and young animals are not changed in a corresponding degree with their mature parents, I might have omitted all the foregoing reasoning and the attempts to explain how this happens; for we might safely have transferred the proposition to the breeds or species naturally selected; and the ultimate effect would necessarily have been that in a number of races or species descended from a common stock and forming several genera and families the embryos would have resembled each other more closely than full-grown animals. Whatever may have been the form or habits of the parent-stock of the Vertebrata, in whatever course the arteries ran and branched, the selection of variations, supervening after the first formation of the arteries in the embryo, would not tend from variations supervening at corresponding periods to alter their course at that period: hence, the similar course of the arteries in the mammal, bird, reptile and fish, must be looked at as a most ancient record of the embryonic structure of the common parent-stock of these four great classes.

{477} Origin, Ed. i. p. 444, vi. p. 611.

A long course of selection might cause a form to become more simple, as well as more complicated; thus the adaptation of a crustaceous{478} animal to live attached during its whole life to the body of a fish, might permit with advantage great simplification of structure, and on this view the singular fact of an embryo being more complex than its parent is at once explained.

{478} Origin, Ed. i. p. 441, vi. p. 607.

On the graduated complexity in each great class.

I may take this opportunity of remarking that naturalists have observed that in most of the great classes a series exists from very complicated to very simple beings; thus in Fish, what a range there is between the sand-eel and shark,—in the Articulata, between the common crab and the Daphnia{479},—between the Aphis and butterfly, and between a mite and a spider{480}. Now the observation just made, namely, that selection might tend to simplify, as well as to complicate, explains this; for we can see that during the endless geologico-geographical changes, and consequent isolation of species, a station occupied in other districts by less complicated animals might be left unfilled, and be occupied by a degraded form of a higher or more complicated class; and it would by no means follow that, when the two regions became united, the degraded organism would give way to the aboriginally lower organism. According to our theory, there is obviously no power tending constantly to exalt species, except the mutual struggle between the different individuals and classes; but from the strong and general hereditary tendency we might expect to find some tendency to progressive complication in the successive production of new organic forms.

{479} Compare Origin, Ed. i. p. 419, vi. p. 575.

{480} Scarcely possible to distinguish between non-development and retrograde development.

Modification by selection of the forms of immature animals.

I have above remarked that the feline{481} form is quite of secondary importance to the embryo and to the kitten. Of course, during any great and prolonged change of structure in the mature animal, it might, and often would be, indispensable that the form of the embryo should be changed; and this could be effected, owing to the hereditary tendency at corresponding ages, by selection, equally well as in mature age: thus if the embryo tended to become, or to remain, either over its whole body or in certain parts, too bulky, the female parent would die or suffer more during parturition; and as in the case of the calves with large hinder quarters{482}, the peculiarity must be either eliminated or the species become extinct. Where an embryonic form has to seek its own food, its structure and adaptation is just as important to the species as that of the full-grown animal; and as we have seen that a peculiarity appearing in a caterpillar (or in a child, as shown by the hereditariness of peculiarities in the milk-teeth) reappears in its offspring, so we can at once see that our common principle of the selection of slight accidental variations would modify and adapt a caterpillar to a new or changing condition, precisely as in the full-grown butterfly. Hence probably it is that caterpillars of different species of the Lepidoptera differ more than those embryos, at a corresponding early period of life, do which remain inactive in the womb of their parents. The parent during successive ages continuing to be adapted by selection for some one object, and the larva for quite another one, we need not wonder at the difference becoming wonderfully great between them; even as great as that between the fixed rock-barnacle and its free, crab-like offspring, which is furnished with eyes and well-articulated, locomotive limbs{483}.

{481} See p. 42, where the same illustration is used.

{482} Var. under Dom., Ed. ii. vol. I. p. 452.

{483} Origin, Ed. i. p. 441, vi. p. 607.

Importance of embryology in classification.

We are now prepared to perceive why the study of embryonic forms is of such acknowledged importance in classification{484}. For we have seen that a variation, supervening at any time, may aid in the modification and adaptation of the full-grown being; but for the modification of the embryo, only the variations which supervene at a very early period can be seized on and perpetuated by selection: hence there will be less power and less tendency (for the structure of the embryo is mostly unimportant) to modify the young: and hence we might expect to find at this period similarities preserved between different groups of species which had been obscured and quite lost in the full-grown animals. I conceive on the view of separate creations it would be impossible to offer any explanation of the affinities of organic beings thus being plainest and of the greatest importance at that period of life when their structure is not adapted to the final part they have to play in the economy of nature.

{484} Origin, Ed. i. p. 449, vi. p. 617.

Order in time in which the great classes have first appeared.

It follows strictly from the above reasoning only that the embryos of (for instance) existing vertebrata resemble more closely the embryo of the parent-stock of this great class than do full-grown existing vertebrata resemble their full-grown parent-stock. But it may be argued with much probability that in the earliest and simplest condition of things the parent and embryo must have resembled each other, and that the passage of any animal through embryonic states in its growth is entirely due to subsequent variations affecting only the more mature periods of life. If so, the embryos of the existing vertebrata will shadow forth the full-grown structure of some of those forms of this great class which existed at the earlier periods of the earth's history{485}: and accordingly, animals with a fish-like structure ought to have preceded birds and mammals; and of fish, that higher organized division with the vertebrae extending into one division of the tail ought to have preceded the equal-tailed, because the embryos of the latter have an unequal tail; and of Crustacea, entomostraca ought to have preceded the ordinary crabs and barnacles—polypes ought to have preceded jelly-fish, and infusorial animalcules to have existed before both. This order of precedence in time in some of these cases is believed to hold good; but I think our evidence is so exceedingly incomplete regarding the number and kinds of organisms which have existed during all, especially the earlier, periods of the earth's history, that I should put no stress on this accordance, even if it held truer than it probably does in our present state of knowledge.

{485} Origin, Ed. i. p. 449, vi. p. 618.



The abortive organs of naturalists.

Parts of structure are said to be "abortive," or when in a still lower state of development "rudimentary{486}," when the same reasoning power, which convinces us that in some cases similar parts are beautifully adapted to certain ends, declares that in others they are absolutely useless. Thus the rhinoceros, the whale{487}, etc., have, when young, small but properly formed teeth, which never protrude from the jaws; certain bones, and even the entire extremities are represented by mere little cylinders or points of bone, often soldered to other bones: many beetles have exceedingly minute but regularly formed wings lying under their wing-cases{488}, which latter are united never to be opened: many plants have, instead of stamens, mere filaments or little knobs; petals are reduced to scales, and whole flowers to buds, which (as in the feather hyacinth) never expand. Similar instances are almost innumerable, and are justly considered wonderful: probably not one organic being exists in which some part does not bear the stamp of inutility; for what can be clearer{489}, as far as our reasoning powers can reach, than that teeth are for eating, extremities for locomotion, wings for flight, stamens and the entire flower for reproduction; yet for these clear ends the parts in question are manifestly unfit. Abortive organs are often said to be mere representatives (a metaphorical expression) of similar parts in other organic beings; but in some cases they are more than representatives, for they seem to be the actual organ not fully grown or developed; thus the existence of mammae in the male vertebrata is one of the oftenest adduced cases of abortion; but we know that these organs in man (and in the bull) have performed their proper function and secreted milk: the cow has normally four mammae and two abortive ones, but these latter in some instances are largely developed and even (??) give milk{490}. Again in flowers, the representatives of stamens and pistils can be traced to be really these parts not developed; Koelreuter has shown by crossing a diaecious plant (a Cucubalus) having a rudimentary pistil{491} with another species having this organ perfect, that in the hybrid offspring the rudimentary part is more developed, though still remaining abortive; now this shows how intimately related in nature the mere rudiment and the fully developed pistil must be.

{486} In the Origin, Ed. i. p. 450, vi. p. 619, the author does not lay stress on any distinction in meaning between the terms abortive and rudimentary organs.

{487} Origin, Ed. i. p. 450, vi. p. 619.

{488} Ibid.

{489} This argument occurs in Origin, Ed. i. p. 451, vi. p. 619.

{490} Origin, Ed. i. p. 451, vi. p. 619, on male mammae. In the Origin he speaks certainly of the abortive mammae of the cow giving milk,—a point which is here queried.

{491} Origin, Ed. i. p. 451, vi. p. 620.

Abortive organs, which must be considered as useless as far as their ordinary and normal purpose is concerned, are sometimes adapted to other ends{492}: thus the marsupial bones, which properly serve to support the young in the mother's pouch, are present in the male and serve as the fulcrum for muscles connected only with male functions: in the male of the marigold flower the pistil is abortive for its proper end of being impregnated, but serves to sweep the pollen out of the anthers{493} ready to be borne by insects to the perfect pistils in the other florets. It is likely in many cases, yet unknown to us, that abortive organs perform some useful function; but in other cases, for instance in that of teeth embedded in the solid jaw-bone, or of mere knobs, the rudiments of stamens and pistils, the boldest imagination will hardly venture to ascribe to them any function. Abortive parts, even when wholly useless to the individual species, are of great signification in the system of nature; for they are often found to be of very high importance in a natural classification{494}; thus the presence and position of entire abortive flowers, in the grasses, cannot be overlooked in attempting to arrange them according to their true affinities. This corroborates a statement in a previous chapter, viz. that the physiological importance of a part is no index of its importance in classification. Finally, abortive organs often are only developed, proportionally with other parts, in the embryonic or young state of each species{495}; this again, especially considering the classificatory importance of abortive organs, is evidently part of the law (stated in the last chapter) that the higher affinities of organisms are often best seen in the stages towards maturity, through which the embryo passes. On the ordinary view of individual creations, I think that scarcely any class of facts in natural history are more wonderful or less capable of receiving explanation.

{492} The case of rudimentary organs adapted to new purposes is discussed in the Origin, Ed. i. p. 451, vi. p. 620.

{493} This is here stated on the authority of Sprengel; see also Origin, Ed. i. p. 452, vi. p. 621.

{494} Origin, Ed. i. p. 455, vi. p. 627. In the margin R. Brown's name is given apparently as the authority for the fact.

{495} Origin, Ed. i. p. 455, vi. p. 626.

The abortive organs of physiologists.

Physiologists and medical men apply the term "abortive" in a somewhat different sense from naturalists; and their application is probably the primary one; namely, to parts, which from accident or disease before birth are not developed or do not grow{496}: thus, when a young animal is born with a little stump in the place of a finger or of the whole extremity, or with a little button instead of a head, or with a mere bead of bony matter instead of a tooth, or with a stump instead of a tail, these parts are said to be aborted. Naturalists on the other hand, as we have seen, apply this term to parts not stunted during the growth of the embryo, but which are as regularly produced in successive generations as any other most essential parts of the structure of the individual: naturalists, therefore, use this term in a metaphorical sense. These two classes of facts, however, blend into each other{497}; by parts accidentally aborted, during the embryonic life of one individual, becoming hereditary in the succeeding generations: thus a cat or dog, born with a stump instead of a tail, tends to transmit stumps to their offspring; and so it is with stumps representing the extremities; and so again with flowers, with defective and rudimentary parts, which are annually produced in new flower-buds and even in successive seedlings. The strong hereditary tendency to reproduce every either congenital or slowly acquired structure, whether useful or injurious to the individual, has been shown in the first part; so that we need feel no surprise at these truly abortive parts becoming hereditary. A curious instance of the force of hereditariness is sometimes seen in two little loose hanging horns, quite useless as far as the function of a horn is concerned, which are produced in hornless races of our domestic cattle{498}. Now I believe no real distinction can be drawn between a stump representing a tail or a horn or the extremities; or a short shrivelled stamen without any pollen; or a dimple in a petal representing a nectary, when such rudiments are regularly reproduced in a race or family, and the true abortive organs of naturalists. And if we had reason to believe (which I think we have not) that all abortive organs had been at some period suddenly produced during the embryonic life of an individual, and afterwards become inherited, we should at once have a simple explanation of the origin of abortive and rudimentary organs{499}. In the same manner as during changes of pronunciation certain letters in a word may become useless{500} in pronouncing it, but yet may aid us in searching for its derivation, so we can see that rudimentary organs, no longer useful to the individual, may be of high importance in ascertaining its descent, that is, its true classification in the natural system.

{496} Origin, Ed. i. p. 454, vi. p. 625.

{497} In the Origin, Ed. i. p. 454, vi. p. 625, the author in referring to semi-monstrous variations adds "But I doubt whether any of these cases throw light on the origin of rudimentary organs in a state of nature." In 1844 he was clearly more inclined to an opposite opinion.

{498} Origin, Ed. i. p. 454, vi. p. 625.

{499} See Origin, Ed. i. p. 454, vi. p. 625. The author there discusses monstrosities in relation to rudimentary organs, and comes to the conclusion that disuse is of more importance, giving as a reason his doubt "whether species under nature ever undergo abrupt changes." It seems to me that in the Origin he gives more weight to the "Lamarckian factor" than he did in 1844. Huxley took the opposite view, see the Introduction.

{500} Origin, Ed. i. p. 455, vi. p. 627.

Abortion from gradual disuse.

There seems to be some probability that continued disuse of any part or organ, and the selection of individuals with such parts slightly less developed, would in the course of ages produce in organic beings under domesticity races with such parts abortive. We have every reason to believe that every part and organ in an individual becomes fully developed only with exercise of its functions; that it becomes developed in a somewhat lesser degree with less exercise; and if forcibly precluded from all action, such part will often become atrophied. Every peculiarity, let it be remembered, tends, especially where both parents have it, to be inherited. The less power of flight in the common duck compared with the wild, must be partly attributed to disuse{501} during successive generations, and as the wing is properly adapted to flight, we must consider our domestic duck in the first stage towards the state of the Apteryx, in which the wings are so curiously abortive. Some naturalists have attributed (and possibly with truth) the falling ears so characteristic of most domestic dogs, some rabbits, oxen, cats, goats, horses, &c., &c., as the effects of the lesser use of the muscles of these flexible parts during successive generations of inactive life; and muscles, which cannot perform their functions, must be considered verging towards abortion. In flowers, again, we see the gradual abortion during successive seedlings (though this is more properly a conversion) of stamens into imperfect petals, and finally into perfect petals. When the eye is blinded in early life the optic nerve sometimes becomes atrophied; may we not believe that where this organ, as is the case with the subterranean mole-like Tuco-tuco {502}, is frequently impaired and lost, that in the course of generations the whole organ might become abortive, as it normally is in some burrowing quadrupeds having nearly similar habits with the Tuco-tuco?

{501} Origin, Ed. i. p. 11, vi. p. 13, where drooping-ears of domestic animals are also given.

{502} Origin, Ed. i. p. 137, vi. p. 170.

In as far then as it is admitted as probable that the effects of disuse (together with occasional true and sudden abortions during the embryonic period) would cause a part to be less developed, and finally to become abortive and useless; then during the infinitely numerous changes of habits in the many descendants from a common stock, we might fairly have expected that cases of organs becom abortive would have been numerous. The preservation of the stump of the tail, as usually happens when an animal is born tailless, we can only explain by the strength of the hereditary principle and by the period in embryo when affected{503}: but on the theory of disuse gradually obliterating a part, we can see, according to the principles explained in the last chapter (viz. of hereditariness at corresponding periods of life{504}, together with the use and disuse of the part in question not being brought into play in early or embryonic life), that organs or parts would tend not to be utterly obliterated, but to be reduced to that state in which they existed in early embryonic life. Owen often speaks of a part in a full-grown animal being in an "embryonic condition." Moreover we can thus see why abortive organs are most developed at an early period of life. Again, by gradual selection, we can see how an organ rendered abortive in its primary use might be converted to other purposes; a duck's wing might come to serve for a fin, as does that of the penguin; an abortive bone might come to serve, by the slow increment and change of place in the muscular fibres, as a fulcrum for a new series of muscles; the pistil{505} of the marigold might become abortive as a reproductive part, but be continued in its function of sweeping the pollen out of the anthers; for if in this latter respect the abortion had not been checked by selection, the species must have become extinct from the pollen remaining enclosed in the capsules of the anthers.

{503} These words seem to have been inserted as an afterthought.

{504} Origin, Ed. i. p. 444, vi. p. 611.

{505} This and similar cases occur in the Origin, Ed. i. p. 452, vi. p. 621.

Finally then I must repeat that these wonderful facts of organs formed with traces of exquisite care, but now either absolutely useless or adapted to ends wholly different from their ordinary end, being present and forming part of the structure of almost every inhabitant of this world, both in long-past and present times—being best developed and often only discoverable at a very early embryonic period, and being full of signification in arranging the long series of organic beings in a natural system—these wonderful facts not only receive a simple explanation on the theory of long-continued selection of many species from a few common parent-stocks, but necessarily follow from this theory. If this theory be rejected, these facts remain quite inexplicable; without indeed we rank as an explanation such loose metaphors as that of De Candolle's{506}, in which the kingdom of nature is compared to a well-covered table, and the abortive organs are considered as put in for the sake of symmetry!

{506} The metaphor of the dishes is given in the Essay of 1842, p. 47, note 3.{Note 173}




I will now recapitulate the course of this work, more fully with respect to the former parts, and briefly the latter. In the first chapter we have seen that most, if not all, organic beings, when taken by man out of their natural condition, and bred during several generations, vary; that is variation is partly due to the direct effect of the new external influences, and partly to the indirect effect on the reproductive system rendering the organization of the offspring in some degree plastic. Of the variations thus produced, man when uncivilised naturally preserves the life, and therefore unintentionally breeds from those individuals most useful to him in his different states: when even semi-civilised, he intentionally separates and breeds from such individuals. Every part of the structure seems occasionally to vary in a very slight degree, and the extent to which all kinds of peculiarities in mind and body, when congenital and when slowly acquired either from external influences, from exercise, or from disuse , is truly wonderful. When several breeds are once formed, then crossing is the most fertile source of new breeds{507}. Variation must be ruled, of course, by the health of the new race, by the tendency to return to the ancestral forms, and by unknown laws determining the proportional increase and symmetry of the body. The amount of variation, which has been effected under domestication, is quite unknown in the majority of domestic beings.

{507} Compare however Darwin's later view:—"The possibility of making distinct races by crossing has been greatly exaggerated," Origin, Ed. i. p. 20, vi. p. 23. The author's change of opinion was no doubt partly due to his experience in breeding pigeons.

In the second chapter it was shown that wild organisms undoubtedly vary in some slight degree: and that the kind of variation, though much less in degree, is similar to that of domestic organisms. It is highly probable that every organic being, if subjected during several generations to new and varying conditions, would vary. It is certain that organisms, living in an isolated country which is undergoing geological changes, must in the course of time be so subjected to new conditions; moreover an organism, when by chance transported into a new station, for instance into an island, will often be exposed to new conditions, and be surrounded by a new series of organic beings. If there were no power at work selecting every slight variation, which opened new sources of subsistence to a being thus situated, the effects of crossing, the chance of death and the constant tendency to reversion to the old parent-form, would prevent the production of new races. If there were any selective agency at work, it seems impossible to assign any limit{508} to the complexity and beauty of the adaptive structures, which might thus be produced: for certainly the limit of possible variation of organic beings, either in a wild or domestic state, is not known.

{508} In the Origin, Ed. i. p. 469, vi. p. 644, Darwin makes a strong statement to this effect.

It was then shown, from the geometrically increasing tendency of each species to multiply (as evidenced from what we know of mankind and of other animals when favoured by circumstances), and from the means of subsistence of each species on an average remaining constant, that during some part of the life of each, or during every few generations, there must be a severe struggle for existence; and that less than a grain{509} in the balance will determine which individuals shall live and which perish. In a country, therefore, undergoing changes, and cut off from the free immigration of species better adapted to the new station and conditions, it cannot be doubted that there is a most powerful means of selection, tending to preserve even the slightest variation, which aided the subsistence or defence of those organic beings, during any part of their whole existence, whose organization had been rendered plastic. Moreover, in animals in which the sexes are distinct, there is a sexual struggle, by which the most vigorous, and consequently the best adapted, will oftener procreate their kind.

{509} "A grain in the balance will determine which individual shall live and which shall die," Origin, Ed. i. p. 467, vi. p. 642. A similar statement occurs in the 1842 Essay, p. 8, note 3.{Note 59}

A new race thus formed by natural selection would be undistinguishable from a species. For comparing, on the one hand, the several species of a genus, and on the other hand several domestic races from a common stock, we cannot discriminate them by the amount of external difference, but only, first, by domestic races not remaining so constant or being so "true" as species are; and secondly by races always producing fertile offspring when crossed. And it was then shown that a race naturally selected—from the variation being slower—from the selection steadily leading towards the same ends{510}, and from every new slight change in structure being adapted (as is implied by its selection) to the new conditions and being fully exercised, and lastly from the freedom from occasional crosses with other species, would almost necessarily be "truer" than a race selected by ignorant or capricious and short-lived man. With respect to the sterility of species when crossed, it was shown not to be a universal character, and when present to vary in degree: sterility also was shown probably to depend less on external than on constitutional differences. And it was shown that when individual animals and plants are placed under new conditions, they become, without losing their healths, as sterile, in the same manner and to the same degree, as hybrids; and it is therefore conceivable that the cross-bred offspring between two species, having different constitutions, might have its constitution affected in the same peculiar manner as when an individual animal or plant is placed under new conditions. Man in selecting domestic races has little wish and still less power to adapt the whole frame to new conditions; in nature, however, where each species survives by a struggle against other species and external nature, the result must be very different.

{510} Thus according to the author what is now known as orthogenesis is due to selection.

Races descending from the same stock were then compared with species of the same genus, and they were found to present some striking analogies. The offspring also of races when crossed, that is mongrels, were compared with the cross-bred offspring of species, that is hybrids, and they were found to resemble each other in all their characters, with the one exception of sterility, and even this, when present, often becomes after some generations variable in degree. The chapter was summed up, and it was shown that no ascertained limit to the amount of variation is known; or could be predicted with due time and changes of condition granted. It was then admitted that although the production of new races, undistinguishable from true species, is probable, we must look to the relations in the past and present geographical distribution of the infinitely numerous beings, by which we are surrounded—to their affinities and to their structure—for any direct evidence.

In the third chapter the inheritable variations in the mental phenomena of domestic and of wild organic beings were considered. It was shown that we are not concerned in this work with the first origin of the leading mental qualities; but that tastes, passions, dispositions, consensual movements, and habits all became, either congenitally or during mature life, modified and were inherited. Several of these modified habits were found to correspond in every essential character with true instincts, and they were found to follow the same laws. Instincts and dispositions &c. are fully as important to the preservation and increase of a species as its corporeal structure; and therefore the natural means of selection would act on and modify them equally with corporeal structures. This being granted, as well as the proposition that mental phenomena are variable, and that the modifications are inheritable, the possibility of the several most complicated instincts being slowly acquired was considered, and it was shown from the very imperfect series in the instincts of the animals now existing, that we are not justified in prima facie rejecting a theory of the common descent of allied organisms from the difficulty of imagining the transitional stages in the various now most complicated and wonderful instincts. We were thus led on to consider the same question with respect both to highly complicated organs, and to the aggregate of several such organs, that is individual organic beings; and it was shown, by the same method of taking the existing most imperfect series, that we ought not at once to reject the theory, because we cannot trace the transitional stages in such organs, or conjecture the transitional habits of such individual species.

In the Second Part{511} the direct evidence of allied forms having descended from the same stock was discussed. It was shown that this theory requires a long series of intermediate forms between the species and groups in the same classes—forms not directly intermediate between existing species, but intermediate with a common parent. It was admitted that if even all the preserved fossils and existing species were collected, such a series would be far from being formed; but it was shown that we have not good evidence that the oldest known deposits are contemporaneous with the first appearance of living beings; or that the several subsequent formations are nearly consecutive; or that any one formation preserves a nearly perfect fauna of even the hard marine organisms, which lived in that quarter of the world. Consequently, we have no reason to suppose that more than a small fraction of the organisms which have lived at any one period have ever been preserved; and hence that we ought not to expect to discover the fossilised sub-varieties between any two species. On the other hand, the evidence, though extremely imperfect, drawn from fossil remains, as far as it does go, is in favour of such a series of organisms having existed as that required. This want of evidence of the past existence of almost infinitely numerous intermediate forms, is, I conceive, much the weightiest difficulty{512} on the theory of common descent; but I must think that this is due to ignorance necessarily resulting from the imperfection of all geological records.

{511} Part II begins with Ch. IV. See the Introduction, where the absence of division into two parts (in the Origin) is discussed.

{512} In the recapitulation in the last chapter of the Origin, Ed. i. p. 475, vi. p. 651, the author does not insist on this point as the weightiest difficulty, though he does so in Ed. i. p. 299. It is possible that he had come to think less of the difficulty in question: this was certainly the case when he wrote the 6th edition, see p. 438.

In the fifth chapter it was shown that new species gradually{513} appear, and that the old ones gradually disappear, from the earth; and this strictly accords with our theory. The extinction of species seems to be preceded by their rarity; and if this be so, no one ought to feel more surprise at a species being exterminated than at its being rare. Every species which is not increasing in number must have its geometrical tendency to increase checked by some agency seldom accurately perceived by us. Each slight increase in the power of this unseen checking agency would cause a corresponding decrease in the average numbers of that species, and the species would become rarer: we feel not the least surprise at one species of a genus being rare and another abundant; why then should we be surprised at its extinction, when we have good reason to believe that this very rarity is its regular precursor and cause.

{513} <The following words:> The fauna changes singly <were inserted by the author, apparently to replace a doubtful erasure>.

In the sixth chapter the leading facts in the geographical distribution of organic beings were considered—namely, the dissimilarity in areas widely and effectually separated, of the organic beings being exposed to very similar conditions (as for instance, within the tropical forests of Africa and America, or on the volcanic islands adjoining them). Also the striking similarity and general relations of the inhabitants of the same great continents, conjoined with a lesser degree of dissimilarity in the inhabitants living on opposite sides of the barriers intersecting it—whether or not these opposite sides are exposed to similar conditions. Also the dissimilarity, though in a still lesser degree, in the inhabitants of different islands in the same archipelago, together with their similarity taken as a whole with the inhabitants of the nearest continent, whatever its character may be. Again, the peculiar relations of Alpine floras; the absence of mammifers on the smaller isolated islands; and the comparative fewness of the plants and other organisms on islands with diversified stations; the connection between the possibility of occasional transportal from one country to another, with an affinity, though not identity, of the organic beings inhabiting them. And lastly, the clear and striking relations between the living and the extinct in the same great divisions of the world; which relation, if we look very far backward, seems to die away. These facts, if we bear in mind the geological changes in progress, all simply follow from the proposition of allied organic beings having lineally descended from common parent-stocks. On the theory of independent creations they must remain, though evidently connected together, inexplicable and disconnected.

In the seventh chapter, the relationship or grouping of extinct and recent species; the appearance and disappearance of groups; the ill-defined objects of the natural classification, not depending on the similarity of organs physiologically important, not being influenced by adaptive or analogical characters, though these often govern the whole economy of the individual, but depending on any character which varies least, and especially on the forms through which the embryo passes, and, as was afterwards shown, on the presence of rudimentary and useless organs. The alliance between the nearest species in distinct groups being general and not especial; the close similarity in the rules and objects in classifying domestic races and true species. All these facts were shown to follow on the natural system being a genealogical system.

In the eighth chapter, the unity of structure throughout large groups, in species adapted to the most different lives, and the wonderful metamorphosis (used metaphorically by naturalists) of one part or organ into another, were shown to follow simply on new species being produced by the selection and inheritance of successive small changes of structure. The unity of type is wonderfully manifested by the similarity of structure, during the embryonic period, in the species of entire classes. To explain this it was shown that the different races of our domestic animals differ less, during their young state, than when full grown; and consequently, if species are produced like races, the same fact, on a greater scale, might have been expected to hold good with them. This remarkable law of nature was attempted to be explained through establishing, by sundry facts, that slight variations originally appear during all periods of life, and that when inherited they tend to appear at the corresponding period of life; according to these principles, in several species descended from the same parent-stock, their embryos would almost necessarily much more closely resemble each other than they would in their adult state. The importance of these embryonic resemblances, in making out a natural or genealogical classification, thus becomes at once obvious. The occasional greater simplicity of structure in the mature animal than in the embryo; the gradation in complexity of the species in the great classes; the adaptation of the larvae of animals to independent powers of existence; the immense difference in certain animals in their larval and mature states, were all shown on the above principles to present no difficulty.

In the chapter, the frequent and almost general presence of organs and parts, called by naturalists abortive or rudimentary, which, though formed with exquisite care, are generally absolutely useless . though sometimes applied to uses not normal,—which cannot be considered as mere representative parts, for they are sometimes capable of performing their proper function,—which are always best developed, and sometimes only developed, during a very early period of life,—and which are of admitted high importance in classification,—were shown to be simply explicable on our theory of common descent.

Why do we wish to reject the theory of common descent?

Thus have many general facts, or laws, been included under one explanation; and the difficulties encountered are those which would naturally result from our acknowledged ignorance. And why should we not admit this theory of descent{514}? Can it be shown that organic beings in a natural state are all absolutely invariable? Can it be said that the limit of variation or the number of varieties capable of being formed under domestication are known? Can any distinct line be drawn between a race and a species? To these three questions we may certainly answer in the negative. As long as species were thought to be divided and defined by an impassable barrier of sterility, whilst we were ignorant of geology, and imagined that the world was of short duration, and the number of its past inhabitants few, we were justified in assuming individual creations, or in saying with Whewell that the beginnings of all things are hidden from man. Why then do we feel so strong an inclination to reject this theory—especially when the actual case of any two species, or even of any two races, is adduced—and one is asked, have these two originally descended from the same parent womb? I believe it is because we are always slow in admitting any great change of which we do not see the intermediate steps. The mind cannot grasp the full meaning of the term of a million or hundred million years, and cannot consequently add up and perceive the full effects of small successive variations accumulated during almost infinitely many generations. The difficulty is the same with that which, with most geologists, it has taken long years to remove, as when Lyell propounded that great valleys{515} were hollowed out [and long lines of inland cliffs had been formed] by the slow action of the waves of the sea. A man may long view a grand precipice without actually believing, though he may not deny it, that thousands of feet in thickness of solid rock once extended over many square miles where the open sea now rolls; without fully believing that the same sea which he sees beating the rock at his feet has been the sole removing power.

{514} This question forms the subject of what is practically a section of the final chapter of the Origin (Ed. i. p. 480, vi. p. 657).

{515} Origin, Ed. i. p. 481, vi. p. 659.

Shall we then allow that the three distinct species of rhinoceros{516} which separately inhabit Java and Sumatra and the neighbouring mainland of Malacca were created, male and female, out of the inorganic materials of these countries? Without any adequate cause, as far as our reason serves, shall we say that they were merely, from living near each other, created very like each other, so as to form a section of the genus dissimilar from the African section, some of the species of which section inhabit very similar and some very dissimilar stations? Shall we say that without any apparent cause they were created on the same generic type with the ancient woolly rhinoceros of Siberia and of the other species which formerly inhabited the same main division of the world: that they were created, less and less closely related, but still with interbranching affinities, with all the other living and extinct mammalia? That without any apparent adequate cause their short necks should contain the same number of vertebrae with the giraffe; that their thick legs should be built on the same plan with those of the antelope, of the mouse, of the hand of the monkey, of the wing of the bat, and of the fin of the porpoise. That in each of these species the second bone of their leg should show clear traces of two bones having been soldered and united into one; that the complicated bones of their head should become intelligible on the supposition of their having been formed of three expanded vertebrae; that in the jaws of each when dissected young there should exist small teeth which never come to the surface. That in possessing these useless abortive teeth, and in other characters, these three rhinoceroses in their embryonic state should much more closely resemble other mammalia than they do when mature. And lastly, that in a still earlier period of life, their arteries should run and branch as in a fish, to carry the blood to gills which do not exist. Now these three species of rhinoceros closely resemble each other; more closely than many generally acknowledged races of our domestic animals; these three species if domesticated would almost certainly vary, and races adapted to different ends might be selected out of such variations. In this state they would probably breed together, and their offspring would possibly be quite, and probably in some degree, fertile; and in either case, by continued crossing, one of these specific forms might be absorbed and lost in another. I repeat, shall we then say that a pair, or a gravid female, of each of these three species of rhinoceros, were separately created with deceptive appearances of true relationship, with the stamp of inutility on some parts, and of conversion in other parts, out of the inorganic elements of Java, Sumatra and Malacca? or have they descended, like our domestic races, from the same parent-stock? For my own part I could no more admit the former proposition than I could admit that the planets move in their courses, and that a stone falls to the ground, not through the intervention of the secondary and appointed law of gravity, but from the direct volition of the Creator.

{516} The discussion on the three species of Rhinoceros which also occurs in the Essay of 1842, p. 48, was omitted in Ch. XIV of the Origin, Ed. i.

Before concluding it will be well to show, although this has incidentally appeared, how far the theory of common descent can legitimately be extended{517}. If we once admit that two true species of the same genus can have descended from the same parent, it will not be possible to deny that two species of two genera may also have descended from a common stock. For in some families the genera approach almost as closely as species of the same genus; and in some orders, for instance in the monocotyledonous plants, the families run closely into each other. We do not hesitate to assign a common origin to dogs or cabbages, because they are divided into groups analogous to the groups in nature. Many naturalists indeed admit that all groups are artificial; and that they depend entirely on the extinction of intermediate species. Some naturalists, however, affirm that though driven from considering sterility as the characteristic of species, that an entire incapacity to propagate together is the best evidence of the existence of natural genera. Even if we put on one side the undoubted fact that some species of the same genus will not breed together, we cannot possibly admit the above rule, seeing that the grouse and pheasant (considered by some good ornithologists as forming two families), the bull-finch and canary-bird have bred together.

{517} This corresponds to a paragraph in the Origin, Ed. i. p. 483, vi. p. 662, where it is assumed that animals have descended "from at most only four or five progenitors, and plants from an equal or lesser number." In the Origin, however, the author goes on, Ed. i. p. 484, vi. p. 663: "Analogy would lead me one step further, namely, to the belief that all animals and plants have descended from some one prototype."

No doubt the more remote two species are from each other, the weaker the arguments become in favour of their common descent. In species of two distinct families the analogy, from the variation of domestic organisms and from the manner of their intermarrying, fails; and the arguments from their geographical distribution quite or almost quite fails. But if we once admit the general principles of this work, as far as a clear unity of type can be made out in groups of species, adapted to play diversified parts in the economy of nature, whether shown in the structure of the embryonic or mature being, and especially if shown by a community of abortive parts, we are legitimately led to admit their community of descent. Naturalists dispute how widely this unity of type extends: most, however, admit that the vertebrata are built on one type; the articulata on another; the mollusca on a third; and the radiata on probably more than one. Plants also appear to fall under three or four great types. On this theory, therefore, all the organisms yet discovered are descendants of probably less than ten parent-forms.


My reasons have now been assigned for believing that specific forms are not immutable creations{518}. The terms used by naturalists of affinity, unity of type, adaptive characters, the metamorphosis and abortion of organs, cease to be metaphorical expressions and become intelligible facts. We no longer look at an organic being as a savage does at a ship{519} or other great work of art, as at a thing wholly beyond his comprehension, but as a production that has a history which we may search into. How interesting do all instincts become when we speculate on their origin as hereditary habits, or as slight congenital modifications of former instincts perpetuated by the individuals so characterised having been preserved. When we look at every complex instinct and mechanism as the summing up of a long history of contrivances, each most useful to its possessor, nearly in the same way as when we look at a great mechanical invention as the summing up of the labour, the experience, the reason, and even the blunders of numerous workmen. How interesting does the geographical distribution of all organic beings, past and present, become as throwing light on the ancient geography of the world. Geology loses glory{520} from the imperfection of its archives, but it gains in the immensity of its subject. There is much grandeur in looking at every existing organic being either as the lineal successor of some form now buried under thousands of feet of solid rock, or as being the co-descendant of that buried form of some more ancient and utterly lost inhabitant of this world. It accords with what we know of the laws impressed by the Creator{521} on matter that the production and extinction of forms should, like the birth and death of individuals, be the result of secondary means. It is derogatory that the Creator of countless Universes should have made by individual acts of His will the myriads of creeping parasites and worms, which since the earliest dawn of life have swarmed over the land and in the depths of the ocean. We cease to be astonished{522} that a group of animals should have been formed to lay their eggs in the bowels and flesh of other sensitive beings; that some animals should live by and even delight in cruelty; that animals should be led away by false instincts; that annually there should be an incalculable waste of the pollen, eggs and immature beings; for we see in all this the inevitable consequences of one great law, of the multiplication of organic beings not created immutable. From death, famine, and the struggle for existence, we see that the most exalted end which we are capable of conceiving, namely, the creation of the higher animals{523}, has directly proceeded. Doubtless, our first impression is to disbelieve that any secondary law could produce infinitely numerous organic beings, each characterised by the most exquisite workmanship and widely extended adaptations: it at first accords better with our faculties to suppose that each required the fiat of a Creator. There{524} is a [simple] grandeur in this view of life with its several powers of growth, reproduction and of sensation, having been originally breathed into matter under a few forms, perhaps into only one{525}, and that whilst this planet has gone cycling onwards according to the fixed laws of gravity and whilst land and water have gone on replacing each other—that from so simple an origin, through the selection of infinitesimal varieties, endless forms most beautiful and most wonderful have been evolved.

{518} This sentence corresponds, not to the final section of the Origin, Ed. i. p. 484, vi. p. 664, but rather to the opening words of the section already referred to (Origin, Ed. i. p. 480, vi. p. 657).

{519} This simile occurs in the Essay of 1842, p. 50, and in the Origin, Ed. i. p. 485, vi. p. 665, i.e. in the final section of Ch. XIV (vi. Ch. XV). In the MS. there is some erasure in pencil of which I have taken no notice.

{520} An almost identical sentence occurs in the Origin, Ed. i. p. 487, vi. p. 667. The fine prophecy (in the Origin, Ed. i. p. 486, vi. p. 666) on "the almost untrodden field of inquiry" is wanting in the present Essay.

{521} See the last paragraph on p. 488 of the Origin, Ed. i., vi. p. 668.

{522} A passage corresponding to this occurs in the sketch of 1842, p. 51, but not in the last chapter of the Origin.

{523} This sentence occurs in an almost identical form in the Origin, Ed. i. p. 490, vi. p. 669. It will be noted that man is not named though clearly referred to. Elsewhere (Origin, Ed. i. p. 488) the author is bolder and writes "Light will be thrown on the origin of man and his history." In Ed. vi. p. 668, he writes "Much light &c."

{524} For the history of this sentence (with which the Origin of Species closes) see the Essay of 1842, p. 52, note 2{Note 184}: also the concluding pages of the Introduction.

{525} These four words are added in pencil between the lines.


For the names of Authors, Birds, Mammals (including names of classes) and Plants, see sub-indexes under Authors, Birds, Mammals and Plants.

Acquired characters, see Characters

Affinities and classification, 35

America, fossils, 177

Analogy, resemblance by, 36, 82, 199, 205, 211

Animals, marine, preservation of as fossils, 25, 139, 141; —marine distribution, 155, 196

Australia, fossils, 177

AUTHORS, NAMES OF:—Ackerman on hybrids, 11; Bakewell, 9, 91; Bateson, W., xxix, 69 n., 217; Bellinghausen, 124; Boitard and Corbie, 106 n.; Brougham, Lord, 17, 117; Brown, R., 233; Buckland on fossils, 24, 137, 145 n.; Buffon on woodpecker, 6; Bunbury (Sir H.), rules for selection, 67; Butler, S., 116 n.; d'Archiac, 146 n.; Darwin, C., origin of his evolutionary views, xi-xv; —on Forbes' theory, 30; —his Journal of Researches quoted, 67 n., 168 n.; —his Cross-and Self-Fertilisation, 69 n., 103 n.; —on crossing Chinese and common goose, 72 n.; Darwin, Mrs, letter to, xxvi; Darwin, F., on Knight's Law, 70 n.; Darwin, R. W., fact supplied by, 42 n., 223; Darwin and Wallace, joint paper by, xxiv, 87 n.; De Candolle, 7, 47, 87, 204, 238; D'Orbigny, 124, 179 n.; Ehrenberg, 146 n.; Ewart on telegony, 108 n.; Falconer, 167; Forbes, E., xxvii, 30, 146 n., 163 n., 165 n.; Gadow, Dr, xxix; Gaertner, 98, 107; Goebel on Knight's Law, 70 n.; Gould on distribution, 156; Gray, Asa, letter to, publication of in Linnean paper explained, xxiv; Henslow, G., on evolution without selection, 63 n.; Henslow, J. S., xxvii; Herbert on hybrids, 12, 98; —sterility of crocus, 99 n.; Hering, 116 n.; Hogg, 115 n.; Holland, Dr, 223; Hooker, J. D., xxvii, xxviii, 153 n.; —on Insular Floras, 161, 164, 167; Huber, P., 118; Hudson on woodpecker, 131 n.; Humboldt, 71, 166; Hunter, W., 114; Hutton, 27, 138; Huxley, 134 n.; —on Darwin, xi, xii, xiv; —on Darwin's Essay of 1844, xxviii, 235; Judd, xi, xiii, xxix, 28, 141 n.; Knight, A., 3 n., 65, 114; —on Domestication, 77; Knight-Darwin Law, 70 n.; Koelreuter, 12, 97, 98, 104, 232; Lamarck, 42 n., 47, 82, 146, 200; —reasons for his belief in mutability, 197; Lindley, 101; Linnean Society, joint paper, see Darwin and Wallace; Linnaeus on sterility of Alpine plants, 101; —on generic characters, 201; Lonsdale, 145 n.; Lyell, xxvii, 134 n., 138, 141 and n., 146 n., 159, 171, 173, 178; —his doctrine carried to an extreme, 26; —his geological metaphor, 27 n., 141; —his uniformitarianism, 53 n.; —his views on imperfection of geological record, 27; Macculloch, 124 n.; Macleay, W. S., 202; Magendie, 117; Malthus, xv, 7, 88, 90; Marr, Dr, xxix; Marshall, 65; —on sheep and cattle, 78 and n.; —on horns of cattle, 207; Mivart, criticisms, 128 n.; Mozart as a child, his skill on the piano compared to instinct, 19 n.; Mueller on consensual movements, 113; —on variation under uniform conditions, (2), 62; —on recapitulation theory, 219; Murchison, 145 n.; Newton, Alfred, 132 n.; Owen, R., xxvii, 219; Pallas, 68, 69; Pennant, 93 n.; Pliny on selection, 67; Poeppig, 113 n.; Prain, Col., xxix; Rengger, sterility, 100; Richardson, 132 n.; Rutherford, H. W., xxix; St Hilaire on races of dogs, 106; —on sterility of tame and domestic animals, 12, 100; Smith, Jordan, 140; Sprengel, 233; Stapf, Dr, xxix; Strickland, xxvii; Suchetet, 97 n.; Thiselton-Dyer, Sir W., xxix, 167; Wallace, xxiv, xxix, 30, 170 n.; Waterhouse, 125, 126; Western, Lord, 9, 65, 91; Whewell, xxviii, 200; Woodward, H. B., 145 n.; Wrangel, 119 n.; Zacharias, Darwin's letter to, xv

Barriers and distribution, 30, 154, 157, 178

Bees, 113, 117; combs of Hive-bee, 19, 121, 125, 126

Beetles, abortive wings of, 45

BIRDS, transporting seeds, 169; feeding young with food different to their own, 19, 126; migration, 123, 124; nests, 120, 121, 122, 126; of Galapagos, 19, 159; rapid increase, 88; song, 117

BIRDS, NAMES OF:—Apteryx, 45, 236; Duck, 46, 61, 65, 128, 224 n.; Fowl, domestic, 59, 82 n., 97, 113, 114, 217; Goose, 72; —periodic habit, 124 n.; Grouse, hybridised, 97, 102; Guinea-fowl, 79; Hawk, sterility, 100; —periodic habit, 124; Opetiorynchus, 83; Orpheus, 31; Ostrich, distribution of, 158; Owl, white barn, 82; Partridge, infertility of, 102; Peacock, 79, 97, 102; Penguin, 128 n., 237; Petrel, 128 n.; Pheasant, 97, 102; Pigeon, 66, 82, 110 n., 113, 114, 116, 117, 129, 135; see Wood-pigeon; Rhea, 158; Robins, increase in numbers, 88, 90; Rock-thrush of Guiana, 93; Swan, species of, 105; Tailor-bird, 18, 118; Turkey, Australian bush-turkey, 121 n., 122; Tyrannus, 31; Water-ouzel, 18 n., 120; Woodcock, loss of migratory instinct, 120; Woodpecker, 6, 16, 128 n., 148; —in treeless lands, 16, 131; Wood-pigeon, 122; Wren, gold-crested, 120; —willow, 105, 148

Breeds, domestic, parentage of, 71

Brothers, death of by same peculiar disease in old age, 42 n., 44 n., 223

Bud variation, 58; see Sports

Butterfly, cabbage, 127

Catastrophes, geological, 145, 147

Caterpillars, food, 126, 127

Characters, acquired, inheritance of, 1, 57, 60, 225; —congenital, 60; —fixed by breeding, 61; —mental, variation in, 17, 112, 119; —running through whole groups, 106; —useless for classification, 199

Cirripedes, 201, 229

Classification, natural system of, 35, 199, 206, 208; —by any constant character, 201; —relation of, to geography, 202; —a law that members of two distinct groups resemble each other not specifically but generally, 203, 212; —of domestic races, 204; —rarity and extinction in relation to, 210

Compensation, law of, 106

Conditions, direct, action of, 1, 57 n., 62, 65; —change of, analogous to crossing, 15, 77 n., 105; —accumulated effects of, 60, 78; —affecting reproduction, 1, 4, 78, 99; —and geographical distribution, 152

Continent originating as archipelago, bearing of on distribution, 189

Cordillera, as channel of migration, 34 n., 191

Correlation, 76

Creation, centres of, 168, 192

Crocodile, 146

Cross-and Self-Fertilisation, early statement of principles of, 15, 69 n., 103 n.

Crossing, swamping effect of, 2, 69, 96; —of bisexual animals and hermaphrodite plants, 2; —analogous to change in conditions, 3, 15, 69; —in relation to breeds, 68; —in plants, adaptations for, 70

Death, feigned by insects, 123

Difficulties, on theory of evolution, 15, 121, 128, 134

Disease, hereditary, 43 n., 58, 222

Distribution, geographical, 29, 31, 151, 174, 177; —in space and time, subject to same laws, 155; —occasional means of (seeds, eggs, &c.), 169

Disuse, inherited effects of, 46, 57

Divergence, principle of, xxv, 37 n., 145 n., 208 n.

Domestication, variation under, 57, 62; —accumulated effects of, 75, 78; —analysis of effects of, 76, 83

Ears, drooping, 236

Elevation, geological, favouring birth of new species, 32, 34 n., 35 n., 185-189; —alternating with subsidence, importance of for evolution, 33, 190; —bad for preservation of fossils, 194

Embryo, branchial arches of, 42, 220; —absence of special adaptation in, 42, 44 n., 220, 228; —less variable than parent, hence importance of embryology for classification, 44 n., 229; —alike in all vertebrates, 42, 218; —occasionally more complicated than adult, 219, 227

Embryology, 42, 218; its value in classification, 45, 200; law of inheritance at corresponding ages, 44 n., 224; young of very distinct breeds closely similar, 44 n., 225

Ephemera, selection falls on larva, 87 n.

Epizoa, 219

Essay of 1842, question as to date of, xvi; description of MS., XX; compared with the Origin, xxii

Essay of 1844, writing of, xvi; compared with that of 1842 and with the Origin, xxii

Evolution, theory of, why do we tend to reject it, 248

Expression, inheritance of, 114

Extinction, 23, 147, 192; locally sudden, 145; continuous with rarity, 147, 198

Extinction and rarity, 198

Eye, 111 n., 128, 129, 130

Faculty, in relation to instinct, 123

Faunas, alpine, 30, 170, 188; of Galapagos, 31 n., 82, 159; insular-alpine very peculiar, 188; insular, 159, 160

Fauna and flora, of islands related to nearest land, 187

Fear of man, inherited, 17, 113

Fertility, interracial, 103, 104

Fish, colours of, 130, 131; eggs of carried by water-beetle, 169; flying, 128 n.; —transported by whirlwind, 169

Floras, alpine, 162; of oceanic islands, 162; alpine, related to surrounding lowlands, 163; alpine, identity of on distant mountains, 163; alpine resembling arctic, 164; arctic relation to alpine, 164

Flower, morphology of, 39, 216; degenerate under domestication if neglected, 58; changed by selection, 66

Fly, causing extinction, 149

Flying, evolution of, 16, 131

Food, causing variations, 1, 58, 77, 78

Formation (geological) evidence from Tertiary system, 144; (geological), groups of species appear suddenly in Secondary, 26, 144; Palaeozoic, if contemporary with beginning of life, author's theory false, 138

Formations, most ancient escape denudation in conditions unfavourable to life, 25, 139

Forms, transitional, 24, 35 n., 136, 142, 194; on rising land, 196; indirectly intermediate, 24, 135

Fossils, Silurian, not those which first existed in the world, 26, 138; falling into or between existing groups and indirectly intermediate, 24, 137; conditions favourable to preservation, not favourable to existence of much life, 25, 139, 141

Fruit, attractive to animals, 130

Galapagos Islands and Darwin's views, xiv; physical character of in relation to fauna, 31 n., 159

Galapagos Islands, fauna, 31 n., 82

Gasteropods, embryology, 218

Genera, crosses between, 11, 97; wide ranging, has wide ranging species, 155; origin of, 209

Geography, in relation to geology, 31 n., 174, 177

Geographical distribution, see Distribution

Geology, as producing changed conditions, 4; evidence from, 22, 133; "destroys geography," 31 n.

Glacial period, effect of on distribution of alpine and arctic plants, 165

Habit in relation to instinct, 17, 113, 115, 116

Habits in animals taught by parent, 18

Heredity, see Inheritance

Homology of limbs, 38, 214

Homology, serial, 39, 215

Hybrid, fowls and grouse, 11; fowl and peacock, 97; pheasant and grouse, 97; Azalea and Rhododendron, 97

Hybrids, gradation in sterility of, 11, 72, 97; sterility of not reciprocal, 97; variability of, 78; compared and contrasted with mongrel, 107

Individual, meaning of term, 58

Inheritance of acquired characters, see Character

Inheritance, delayed or latent, 43, 44 n., 223; of character at a time of life corresponding to that at which it first appeared, 43, 44 n., 223; germinal, 44, 222, 223

Insect, adapted to fertilise flowers, 87; feigning death, 123; metamorphosis, 129; variation in larvae, 223

Instinct, variation in, 17, 112; and faculty, 18, 123; guided by reason, 18, 19, 118; migratory, 19; migratory, loss of by woodcocks, 120; migratory, origin of, 125; due to germinal variation rather than habit, 116; requiring education for perfection, 117; characterised by ignorance of end: e.g. butterflies laying eggs, 17, 118; butterflies laying eggs on proper plant, 118, 127; instinct, natural selection applicable to, 19, 120

Instinct, for finding the way, 124; periodic, i.e. for lapse of time, 124; comb-making of bee, 125; birds feeding young, 19, 126; nest-building, gradation in, 18, 120, 121, 122; instincts, complex, difficulty in believing in their evolution, 20, 121

Intermediate forms, see Forms

Island, see Elevation, Fauna, Flora

Island, upheaved and gradually colonised, 184

Islands, nurseries of new species, 33, 35 n., 185, 189

Isolation, 32, 34 n., 64, 95, 183, 184

Lepidosiren, 140 n., 212

Limbs, vertebrate, of one type, 38, 216

MAMMALS, arctic, transported by icebergs, 170; distribution, 151, 152, 193; distribution of, ruled by barriers, 154; introduced by man on islands, 172; not found on oceanic islands, 172; relations in time and space, similarity of, 176; of Tertiary period, relation of to existing forms in same region, 174

MAMMALS, NAMES OF:— Antelope, 148; Armadillo, 174; Ass, 79, 107, 172; Bat, 38, 123, 128 n., 131, 132, 214; Bear, sterile in captivity, 100; —whale-like habit, 128 n.; Bizcacha, 168, 203, 212; Bull, mammae of, 232; Carnivora, law of compensation in, 106; Cats, run wild at Ascension, 172; —tailless, 60; Cattle, horns of, 75, 207; —increase in S. America, 90; —Indian, 205; —Niata, 61, 73; —suffering in parturition from too large calves, 75; Cheetah, sterility of, 100 and n.; Chironectes, 199; Cow, abortive mammae, 232; Ctenomys, see Tuco-tuco; Dog, 106, 114; —in Cuba, 113 and n.; —mongrel breed in oceanic islands, 70; —difference in size a bar to crossing, 97; —domestic, parentage of, 71, 72, 73; —drooping ears, 236; —effects of selection, 66; —inter-fertile, 14; —long-legged breed produced to catch hares, 9, 10, 91, 92; —of savages, 67; —races of resembling genera, 106, 204; —Australian, change of colour in, 61; —bloodhound, Cuban, 204; —bull-dog, 113; —foxhound, 114, 116; —greyhound and bull-dog, young of resembling each other, 43, 44 n., 225; —pointer, 114, 115, 116, 117, 118; —retriever, 118 n.; —setter, 114; —shepherd-dog and harrier crossed, instinct of, 118, 119; —tailless, 60; —turnspit, 66; Echidna, 82 n.; Edentata, fossil and living in S. America, 174; Elephant, sterility of, 12, 100; Elk, 125; Ferret, fertility of, 12, 102; Fox, 82, 173, 181; Galeopithecus, 131 n.; Giraffe, fossil, 177; —tail, 128 n.; Goat, run wild at Tahiti, 172; Guanaco, 175; Guinea-pig, 69; Hare, S. American, 158 n.; Hedgehog, 82 n.; Horse, 67, 113, 115, 148, 149; —checks to increase, 148, 149; —increase in S. America, 90; —malconformations and lameness inherited, 58; —parentage, 71, 72; —stripes on, 107; —young of cart-horse and racehorse resembling each other, 43; Hyena, fossil, 177; Jaguar, catching fish, 132; Lemur, flying, 131 n.; Macrauchenia, 137; Marsupials, fossil in Europe, 175 n., 177; —pouch bones, 232, 237; Mastodon, 177; Mouse, 153, 155; —enormous rate of increase, 89, 90; Mule, occasionally breeding, 97, 102; Musk-deer, fossil, 177; Mustela vison, 128 n., 132 n.; Mydas, 170; Mydaus, 170; Nutria, see Otter; Otter, 131, 132, 170; —marsupial, 199, 205, 211; Pachydermata, 137; Phascolomys, 203, 212; Pig, 115, 217; —in oceanic islands, 70; —run wild at St Helena, 172; Pole-cat, aquatic, 128 n., 132 n.; Porpoise, paddle of, 38, 214; Rabbit, 74, 113, 236; Rat, Norway, 153; Reindeer, 125; Rhinoceros, 148; —abortive teeth of, 45, 231; —three oriental species of, 48, 249; Ruminantia, 137 and n.; Seal, 93 n., 131; Sheep, 68, 78, 117, 205; —Ancon variety, 59, 66, 73; —inherited habit of returning home to lamb, 115; —transandantes of Spain, their migratory instinct, 114, 117, 124 n.; Squirrel, flying, 131; Tapir, 135, 136; Tuco-tuco, blindness of, 46, 236; Whale, rudimentary teeth, 45, 229; Wolf, 71, 72, 82; Yak, 72

Metamorphosis, literal not metaphorical, 41, 217

Metamorphosis, e.g. leaves into petals, 215

Migrants to new land, struggle among, 33, 185

Migration, taking the place of variation, 188

Monstrosities, as starting-points of breeds, 49, 59; their relation to rudimentary organs, 46, 234

Morphology, 38, 215; terminology of, no longer metaphorically used, 41, 217

Mutation, see Sports

Natural selection, see Selection

Nest, bird's, see Instinct

Ocean, depth of, and fossils, 25, 195

Organisms, gradual introduction of new, 23, 144; extinct related to, existing in the same manner as representative existing ones to each other, 33, 192; introduced, beating indigenes, 153; dependent on other organisms rather than on physical surroundings, 185; graduated complexity in the great classes, 227; immature, how subject to natural selection, 42, 220, 228; all descended from a few parent-forms, 52, 252

Organs, perfect, objection to their evolution, 15, 128; distinct in adult life, indistinguishable in embryo, 42, 218; rudimentary, 45, 231, 232, 233; rudimentary, compared to monstrosities, 46, 234; rudimentary, caused by disuse, 46, 235; rudimentary, adapted to new ends, 47, 237

Orthogenesis, 241 n.

Oscillation of level in relation to continents, 33, 34 n., 189

Pallas, on parentage of domestic animals, 71

Pampas, imaginary case of farmer on, 32, 184

Perfection, no inherent tendency towards, 227

Plants, see also Flora; fertilisation, 70; migration of, to arctic and antarctic regions, 167; alpine and arctic, migration of, 31, 166; alpine, characters common to, 162; alpine, sterility of, 13, 101

PLANTS, NAMES OF:—AEgilops, 58 n.; Artichoke (Jerusalem), 79; Ash, weeping, seeds of, 61; Asparagus, 79; Azalea, 13, 59, 97; Cabbage, 109, 135, 204; Calceolaria, 11, 99; Cardoon, 153; Carrot, variation of, 58 n.; Chrysanthemum, 59; Crinum, 11, 99; Crocus, 96, 99 n.; Cucubalus, crossing, 232; Dahlia, 21, 59, 63, 69, 74, 110; Foxglove, 82; Gentian, colour of flower, 107 n.; Geranium, 102; Gladiolus, crossed, ancestry of, 11; Grass, abortive flowers, 233; Heath, sterility, 96; Hyacinth, colours of, 106; —feather-hyacinth, 229; Juniperus, hybridised, 97; Laburnum, peculiar hybrid, 108; Lilac, sterility of, 13, 100; Marigold, style of, 47, 233, 237; Mistletoe, 6, 86, 87, 90 n.; Nectarines on peach trees, 59; Oxalis, colour of flowers of, 107 n.; Phaseolus, cultivated form suffers from frost, 109; Pine-apple, 207; Poppy, Mexican, 154; Potato, 69, 74, 110; Rhododendron, 97, 99; Rose, moss, 59; —Scotch, 69; Seakale, 79; Sweet-william, 59; Syringa, persica and chinensis, see Lilac; Teazle, 129; Thuja, hybridised, 97; Tulips, "breaking" of, 58; Turnip, Swedish and common, 205; Vine, peculiar hybrid, 108; Yew, weeping, seeds of, 61

Plasticity, produced by domestication, 1, 63

Plesiosaurus, loss of unity of type in, 41, 217

Pteropods, embryology, 218

Quadrupeds, extinction of large, 147

Quinary System, 202

Race, the word used as equivalent to variety, 94

Races, domestic, classification of, 204

Rarity, 28, 148; and extinction, 28, 149, 210

Recapitulation theory, 42, 219, 230, 239

Record, geological, imperfection of, 26, 140

Regions, geographical, of the world, 29, 152, 174; formerly less distinct as judged by fossils, 177

Resemblance, analogical, 36, 199

Reversion, 3, 64, 69, 74

"Roguing," 65

Rudimentary organs, see Organs

Savages, domestic animals of, 67, 68, 96

Selection, human, 3, 63; references to the practice of, in past times, 67; great effect produced by, 3, 91; necessary for the formation of breeds, 64; methodical, effects of, 3, 65; unconscious, 3, 67

Selection, natural, xvi, 7, 87; natural compared to human, 85, 94, 224; of instincts, 19, 120; difficulty of believing, 15, 121, 128

Selection, sexual, two types of, 10, 92

Silk-worms, variation in larval state, 44 n., 223

Skull, morphology of, 39, 215

Species, representative, seen in going from N. to S. in a continent, 31 n., 156; representative in archipelagoes, 187; wide-ranging, 34 n., 146; and varieties, difficulty of distinguishing, 4, 81, 197; sterility of crosses between, supposed to be criterion, 11, 134; gradual appearance and disappearance of, 23, 144; survival of a few among many extinct, 146

Species, not created more than once, 168, 171, 191; evolution of, compared to birth of individuals, 150, 198, 253; small number in New Zealand as compared to the Cape, 171, 191; persistence of, unchanged, 192, 199

Sports, 1, 58, 59, 64, 74, 95, 129, 186, 206, 224

Sterility, due to captivity, 12, 77 n., 100; of various plants, 13, 101; of species when crossed, 11, 23, 96, 99, 103; produced by conditions, compared to sterility due to crossing, 101, 102

Struggle for life, 7, 91, 92, 148, 241

Subsidence, importance of, in relation to fossils, 25, 35 n., 195; of continent leading to isolation of organisms, 190; not favourable to birth of new species, 196

Swimming bladder, 16, 129

System, natural, is genealogical, 36, 208

Telegony, 108

Tibia and fibula, 48, 137

Time, enormous lapse of, in geological epochs, 25, 140

Tortoise, 146

Transitional forms, see Forms

Trigonia, 147 n., 199

Tree-frogs in treeless regions, 131

Type, unity of, 38, 214; uniformity of, lost in Plesiosaurus, 217; persistence of, in continents, 158, 178

Uniformitarian views of Lyell, bearing on evolution, 249

Use, inherited effects of, see Characters, acquired

Variability, as specific character, 83; produced by change and also by crossing, 105

Variation, by Sports, see Sports; under domestication, 1, 57, 63, 78; due to causes acting on reproductive system, see Variation, germinal; —germinal, 2, 43, 62, 222; individual, 57 n.; causes of, 1, 4, 57, 61; due to crossing, 68, 69; limits of, 74, 75, 82, 109; small in state of nature, 4, 59 n., 81, 83; results of without selection, 84; —minute, value of, 91; analogous in species of same genus, 107; of mental attributes, 17, 112; in mature life, 59, 224, 225

Varieties, minute, in birds, 82; resemblance of to species, 81 n., 82, 105

Vertebrate skull, morphology of, 215

Wildness, hereditary, 113, 119


Transcriber's Notes & Errata Inline transcriber's notes are enclosed in curly brackets. Footnote anchors and labels are enclosed in curly brackets. The footnotes have been renumbered consecutively. Because of this, the changed footnote numbers are appended in curly brackets to the internal cross-references. Superscript letters are denoted by a preceding caret e.g., d^o 'oe' ligatures have been rendered as separate letters. The following typographical errors have been corrected. simplication simplification care case apparant apparent The following words were found in both hyphenated and unhyphenated forms. The figures in parentheses are the number of instances of each. after-thought (1) afterthought (2) blood-hound (2) bloodhound (1) bull-dog (7) bulldog (2) co-descendants (1) codescendants (1) feather-hyacinth (2) feather hyacinth (1) grey-hound (2) greyhound (10) high-lands (3) highlands (2) long-legged (2) long legged (1) race-horse (2) racehorse (4) shepherd-dog (3) shepherd dog (1) sub-divisions (3) subdivisions (4) table-land (2) tableland (1)


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