{296} "and that each eye throughout the animal kingdom is not only most useful, but _perfect_ for its possessor."{297} _Origin_, Ed. i. p. 190, vi. p. 230.
{298} This is one of the most definite statements in the present Essay of the possible importance of _sports_ or what would now be called _mutations_. As is well known the author afterwards doubted whether species could arise in this way. See _Origin_, Ed. v. p.
103, vi. p. 110, also _Life and Letters_, vol. iii. p. 107.
The nature or condition of certain structures has been thought by some naturalists to be of no use to the possessor{299}, but to have been formed wholly for the good of other species; thus certain fruit and seeds have been thought to have been made nutritious for certain animals--numbers of insects, especially in their larval state, to exist for the same end--certain fish to be bright coloured to aid certain birds of prey in catching them, &c. Now could this be proved (which I am far from admitting) the theory of natural selection would be quite overthrown; for it is evident that selection depending on the advantage over others of one individual with some slight deviation would never produce a structure or quality profitable only to another species. No doubt one being takes advantage of qualities in another, and may even cause its extermination; but this is far from proving that this quality was produced for such an end. It may be advantageous to a plant to have its seeds attractive to animals, if one out of a hundred or a thousand escapes being digested, and thus aids dissemination: the bright colours of a fish may be of some advantage to it, or more probably may result from exposure to certain conditions in favourable haunts for food, _notwithstanding_ it becomes subject to be caught more easily by certain birds.
{299} See _Origin_, Ed. i. p. 210, vi. p. 322, where the question is discussed for the case of instincts with a proviso that the same argument applies to structure. It is briefly stated in its general bearing in _Origin_, Ed. i. p. 87, vi. p. 106.
If instead of looking, as above, at certain individual organs, in order to speculate on the stages by which their parts have been matured and selected, we consider an individual animal, we meet with the same or greater difficulty, but which, I believe, as in the case of single organs, rests entirely on our ignorance. It may be asked by what intermediate forms could, for instance, a bat possibly have pa.s.sed; but the same question might have been asked with respect to the seal, if we had not been familiar with the otter and other semi-aquatic carnivorous quadrupeds. But in the case of the bat, who can say what might have been the habits of some parent form with less developed wings, when we now have insectivorous opossums and herbivorous squirrels fitted for merely gliding through the air{300}. One species of bat is at present partly aquatic in its habits{301}. Woodp.e.c.k.e.rs and tree-frogs are especially adapted, as their names express, for climbing trees; yet we have species of both inhabiting the open plains of La Plata, where a tree does not exist{302}. I might argue from this circ.u.mstance that a structure eminently fitted for climbing trees might descend from forms inhabiting a country where a tree did not exist. Notwithstanding these and a mult.i.tude of other well-known facts, it has been maintained by several authors that one species, for instance of the carnivorous order, could not pa.s.s into another, for instance into an otter, because in its transitional state its habits would not be adapted to any proper conditions of life; but the jaguar{303} is a thoroughly terrestrial quadruped in its structure, yet it takes freely to the water and catches many fish; will it be said that it is _impossible_ that the conditions of its country might become such that the jaguar should be driven to feed more on fish than they now do; and in that case is it impossible, is it not probable, that any the slightest deviation in its instincts, its form of body, in the width of its feet, and in the extension of the skin (which already unites the base of its toes) would give such individuals a better _chance_ of surviving and propagating young with similar, barely perceptible (though thoroughly exercised), deviations{304}? Who will say what could thus be effected in the course of ten thousand generations? Who can answer the same question with respect to instincts? If no one can, the _possibility_ (for we are not in this chapter considering the _probability_) of simple organs or organic beings being modified by natural selection and the effects of external agencies into complicated ones ought not to be absolutely rejected.
{300} No one will dispute that the gliding is most useful, probably necessary for the species in question.{301} Is this the Galeopithecus? I forget.<_galeopithecus_ _origin_="" among="" aquatic="" as="" bats.="" corresponding="" described="" discussion="" do="" ed.="" flying="" formerly="" habits.="" i="" i.="" in="" is="" it="" its="" know="" lemur="" mentioned="" not="" p.="" partly="" placed="" the="" vi.="" why="">
{302} In the _Origin_, Ed. vi. p. 221, the author modified the statement that it _never_ climbs trees; he also inserted a sentence quoting Mr Hudson to the effect that in other districts this woodp.e.c.k.e.r climbs trees and bores holes. See Mr Darwin"s paper, _Zoolog. Soc. Proc._, 1870, and _Life and Letters_, iii. p. 153.
{303} Note by the late Alfred Newton. Richardson in _Fauna Boreali-Americana_, i. p. 49.
{304} See Richardson a far better case of a polecat animal <_mustela vison_="">, which half-year is aquatic. PART II{305}
ON THE EVIDENCE FAVOURABLE AND OPPOSED TO THE VIEW THAT SPECIES ARE NATURALLY FORMED RACES, DESCENDED FROM COMMON STOCKS
{305} In the _Origin_ the division of the work into Parts I and II is omitted. In the MS. the chapters of Part II are numbered afresh, the present being Ch. I of Pt. II. I have thought it best to call it Ch. IV and there is evidence that Darwin had some thought of doing the same. It corresponds to Ch. IX of _Origin_, Ed. i., Ch. X in Ed. vi.
CHAPTER IV
ON THE NUMBER OF INTERMEDIATE FORMS REQUIRED ON THE THEORY OF COMMON DESCENT; AND ON THEIR ABSENCE IN A FOSSIL STATE
I must here premise that, according to the view ordinarily received, the myriads of organisms, which have during past and present times peopled this world, have been created by so many distinct acts of creation. It is impossible to reason concerning the will of the Creator, and therefore, according to this view, we can see no cause why or why not the individual organism should have been created on any fixed scheme.
That all the organisms of this world have been produced on a scheme is certain from their general affinities; and if this scheme can be shown to be the same with that which would result from allied organic beings descending from common stocks, it becomes highly improbable that they have been separately created by individual acts of the will of a Creator. For as well might it be said that, although the planets move in courses conformably to the law of gravity, yet we ought to attribute the course of each planet to the individual act of the will of the Creator{306}. It is in every case more conformable with what we know of the government of this earth, that the Creator should have imposed only general laws. As long as no method was known by which races could become exquisitely adapted to various ends, whilst the existence of species was thought to be proved by the sterility{307} of their offspring, it was allowable to attribute each organism to an individual act of creation.
But in the two former chapters it has (I think) been shown that the production, under existing conditions, of exquisitely adapted species, is at least _possible_. Is there then any direct evidence in favour or against this view? I believe that the geographical distribution of organic beings in past and present times, the kind of affinity linking them together, their so-called "metamorphic" and "abortive" organs, appear in favour of this view. On the other hand, the imperfect evidence of the continuousness of the organic series, which, we shall immediately see, is required on our theory, is against it; and is the most weighty objection{308}. The evidence, however, even on this point, as far as it goes, is favourable; and considering the imperfection of our knowledge, especially with respect to past ages, it would be surprising if evidence drawn from such sources were not also imperfect.{306} In the Essay of 1842 the author uses astronomy in the same manner as an ill.u.s.tration. In the _Origin_ this does not occur; the reference to the action of secondary causes is more general, _e.g._ Ed. i. p. 488, vi. p. 668.
{307} It is interesting to find the argument from sterility given so prominent a place. In a corresponding pa.s.sage in the _Origin_, Ed. i. p. 480, vi. p. 659, it is more summarily treated. The author gives, as the chief bar to the acceptance of evolution, the fact that "we are always slow in admitting any great change of which we do not see the intermediate steps"; and goes on to quote Lyell on geological action. It will be remembered that the question of sterility remained a difficulty for Huxley.
{308} Similar statements occur in the Essay of 1842, p. 24, note 1, and in the _Origin_, Ed. i. p. 299.
As I suppose that species have been formed in an a.n.a.logous manner with the varieties of the domesticated animals and plants, so must there have existed intermediate forms between all the species of the same group, not differing more than recognised varieties differ. It must not be supposed necessary that there should have existed forms exactly intermediate in character between any two species of a genus, or even between any two varieties of a species; but it is necessary that there should have existed every intermediate form between the one species or variety of the common parent, and likewise between the second species or variety, and this same common parent. Thus it does not necessarily follow that there ever has existed