Natural Selection is most destructive of those birds which have departed most from the ideal type, and its activity raises the general standard by favouring those birds which approach the structural ideal.
Variation supplies the raw material upon which Natural Selection is supposed to work. When we turn to examine the process by which its results should be produced, we find, quite apart from the above difficulties, a crop of others still more formidable.
It must be remembered, that the variations on which Natural Selection must work are in each instance extremely minute, well-nigh infinitesimal. Mr. Darwin was as strongly opposed to the idea of Nature making sudden bounds, as to that of a predetermined course of development. But, he argued, an extra chance of living, however slight, must necessarily tell in the long run, the theory of probabilities giving results as certain as any others in mathematics, and, according to these, we may confidently say that, given sufficient time, the[{166}] favoured individuals would infallibly distance their competitors.
The impressiveness of such an argument depends upon its seemingly mathematical character, which is however wholly fallacious, for the probabilities are all the other way. It is perfectly true that a beneficial variation however slight will confer on its happy possessor a corresponding advantage in the struggle for life, as compared with each individual of the non-favoured herd, but, as to that herd collectively, the chances would, on the contrary, ensure that some of its members should outlive the favoured one. Let us even imagine the advantage of the latter to be very great, great enough to double his chances, so that the odds on his surviving each of his fellows will be two to one. Yet if there be a dozen of them to contend with, the odds will be six to one against his surviving the lot. And what of the actual case of minutest benefits conferred by variation? In order to give them even an equal chance of survival, the numbers of those possessing such advantages must be large in proportion as the advantages themselves are small. Thus, if a variation increases the chance of life by one-thousandth part, so that the odds on its possessor are 1001, against 1000 on each non-possessor, yet unless the number of possessors be to that of non-possessors as 1,000 to 1,001, their collective chances will not even be equal. As it is quite absurd to suppose that casual variations could ever occur in such wholesale fashion, how can it[{167}] be supposed that, were Natural Selection the only factor operating, minute advantages could be accumulated by variation even in the simplest cases?
But it is also hard to suppose that in any actual case is the matter so simple as it appears to our limited comprehension. To take for instance the above example of the giraffe. It is very well to have a neck that will reach high-branches of a tree,—but this is not everything. For the mere prolongation of life, much else is required, fleet limbs to distance lions, and keen senses, sight, hearing, and smell, to give warning of the approach of human or other hunters, to say nothing of the extra strengthening of muscles and bones which increased size and weight demands. Unless, however, improvements in all these respects happened casually to concur in the same individual, which could scarcely happen, it is clear that each would militate against the others, for the survival of an individual beneficially developed in one respect, would tend to the extinction of other beneficial developments, possessed by individuals whom he overcame in the struggle for life.
Even the case of the insular insects is by no means so plain as might at first sight appear. There can be no doubt that wings are of some advantage, or on no system could they be supposed to exist. Nor do their advantages cease because disadvantages outweigh them. If some insects are blown out to sea when flying, others will doubtless perish in one way or another because they cannot fly. It may[{168}] even be that those which can fly best will survive, as being able to make head against a breeze which overpowers others. Natural Selection will thus have many arrows in its quiver, some of which must reach the wrong objects.
Still more clearly does this appear in the case of complex structures in which, if they were produced as Mr. Darwin supposes, variation must have hit simultaneously upon independent contrivances, without each of which all the others would be useless and confer no benefit at all. In the eye, for example, to mention but one or two of innumerable similar points, it would be of no avail to have a retina, even such as has been described, without a lens to throw an image upon it, set just at the proper distance, and provided with muscles to alter its shape according to the distance of the object. How can Natural Selection be even conceived to have set to work on such a task as this?
It is still more fundamental to observe that, according to Mr. Darwin's own showing, Natural Selection is purely negative in its action. "If it does select, it selects for death and not for life."[197] It can originate nothing, but only destroy. All that it does for favoured races is to spare them while it sweeps away others, and the sole benefit they derive from it is to have more ample resources upon which to draw. But as for anything they possess in the way of structure or character, they must derive it entirely from themselves—Natural Selection can no more confer it, than the labourer who weeds a garden bed makes the flowers that grow there. Let[{169}] it be imagined that the first human beings on earth, any number of thousand years ago, planted a garden, and determined to produce a rose, by eliminating every plant that did not show some promise of progress rose-wards. Let the gardeners have been endowed with acumen sufficient to detect every symptom of such a tendency, and let their operations have been carried on without interruption to this day,—it is obvious that if roses had resulted, it could only be because among the plants they allowed to remain there existed a rose-making quality of some kind, to which, and not to anything done by human art or skill, the result was due. It would likewise have to be supposed that there were infinite other potentialities latent in the original plants, as of evolving thistles, shamrocks, or leeks—all equally awaiting their opportunity. Selective action could effectually put such competitors out of the way; but in the way of developing a race it could but leave it entirely to itself. Precisely similar is the part played by Natural Selection, except that it must needs play it immensely more slowly,—and if no one can fancy that human agency could by any possibility grow roses unless from some stock predetermined to grow into a rose and nothing else, what grounds have we that can be called scientific for attributing to a blind struggle for life an incomparably greater potency? Nor does it avail to quote the immense extent of time which may be supposed to have been available. No more than Natural Selection has[{170}] time by itself any creative power. We know on the contrary by experience, that when things are not controlled by some principle of order, the lapse of time serves only to make confusion worse confounded.
Another consideration of prime importance is too frequently ignored. On Darwinian principles, each step in any development can be made, not because it leads to an advantageous result in the future, but only because it is itself advantageous. At each stage favoured individuals survive others because they are favoured here and now, not because, when the development they promote shall be completed, their remote descendants will be favoured. Hence it must, for instance, be possible to suppose, that all the intermediate forms between two extremes, whereof one is supposed to have originated the other, were, each in its day, so beneficial as to preserve their possessors at the expense of non-possessors. But can this possibly be even imagined?
To take one example. We have heard, speaking of embryology, that the feet of lizards and the wings and feet of birds arise from the same fundamental form of limb, whence it is argued that birds and lizards are alike descended from a common sauroid, or lizard-like, ancestor, whose limbs in the case of the former class have developed into wings and into feet of a totally new type,—while scales were developing into feathers, and innumerable alterations of internal structure were simultaneously in progress. But[{171}] if so, to confine our attention to one particular, it must be true that each of the innumerable minute gradations between the fore-limb of a lizard and the wing of a bird, was in its turn the best kind of member for a creature to possess, giving him a distinct advantage in the struggle for existence. Nothing, however, appears plainer than that this could not possibly have been the case. The limb shaping towards a wing would be a very clumsy and inefficient leg long before it got to the point at which it became of the slightest use for purposes of flight, that is to say before its alteration was accompanied by any utility whatever. We can neither imagine that creatures furnished with limbs of such intermediate forms could have been otherwise than hopelessly handicapped by them, nor do we find anywhere in the rocks any trace whatever of the innumerable series of modifications which would be needed to link by imperceptible gradations legs and wings together.