4. Herbert Spencer urges, as an objection to the theory of natural selection, that favourable variations in one organ are likely to be counterbalanced by unfavourable variations in some other organ. He maintains that the chances are enormous against the occurrence of the “many coincident and co-ordinated variations” that are necessary to create a life or death determining advantage.
This objection was urged by a writer in the Edinburgh Review in January 1909, and even by Wallace himself in the Fortnightly Review last March against the mutation theory. This objection, strong though it appears on paper, exists only in the imagination of the objector.
Those who urge it display a misunderstanding of the manner in which natural selection acts, and ignorance of the phenomenon of the correlation of organs.
Correlation
Natural selection deals with an organism as a whole. Its effect is to permit those creatures to survive which, taken as a whole, are best adapted to their environment.
Physiologists insist with ever-increasing emphasis that there is more or less correlation and inter-connection between the various parts of an organism.
The several organs of an animal are not so many isolated units. It is impossible to act on one organ without affecting some or all of the others.
Variations in a given direction of one organ are usually accompanied by correlated variations in some of the other organs. If strength be of paramount importance to an animal, natural selection will tend to preserve those individuals which exhibit strength to a marked degree, and this exhibition of strength may be accompanied by other peculiarities, such as short legs or a certain colour, so that natural selection will indirectly tend to produce individuals with short legs and having the colour in question, and it may happen that this particular colour is one that renders the animal more conspicuous than the normal colour does. Nevertheless, on account of the all-needful strength which accompanies it, those animals so coloured may survive while those of a more protective hue perish. Thus, paradoxical though it seems, natural selection may indirectly be responsible for characteristics which in themselves are injurious to the individual. This is probably the case as regards the decorative plumage of some male birds. The phenomenon of correlation was recognised by Darwin, and has, we believe, played an important part in the making of species. We shall deal more fully with the subject in a later chapter.
5. An oft-urged objection to the theory of natural selection, and one which weighed very strongly with Huxley, is that breeders have hitherto not succeeded in breeding a variety which is infertile with the parent species. If, Huxley asked, breeders cannot produce such a thing, how can we say we consider it proved that natural selection produces new species in nature? This objection, however, loses much of its force in view of the fact that many perfectly distinct species are quite fertile when bred together. We shall recur to this in Chapter IV.
6. The fact that palæontology has hitherto failed to yield links connecting many existing species is a classical objection to the theory of the origin of species by gradual evolution.