In the first place, it must be evident that so soon as we cease to be bound by any a priori deduction as to natural selection being "the exclusive means of modifications," it ceases to be a matter of much concern to the theory of natural selection in what proportion other means of modification have been at work—especially when non-adaptive modifications are concerned, and where these have reference to merely "specific characters," or modifications of the most incipient kind, least generally diffused among organic types, and representing the incidence of causes of less importance than any others in the process of organic evolution considered as a whole. Consequently, in the second place, we find that Darwin nowhere displays any solicitude touching the proportional number of specific characters that may eventually prove to be due to causes other than natural selection. He takes a much wider and deeper view of organic evolution, and, having entirely emancipated himself from the former conception of species as the organic units, sees virtually no significance in specific characters, except in so far as they are also adaptive characters.
Such, at all events, appears to me the obvious interpretation of his writings when these are carefully read with a view to ascertaining his ideas upon "Utilitarian doctrine: how far true." And I make these remarks because it has been laid to my charge, that in quoting such passages as the above I have been putting "a strained interpretation" upon Darwin's utterances: "such admissions," it is said, "Mr. Romanes appears to me to treat as if wrung from a hostile witness[167]." But, from what has gone before, it ought to be apparent that I take precisely the opposite view to that here imputed. Far from deeming these and similar passages as "admissions wrung from a hostile witness," and far from seeking to put any "strained interpretation" upon them, I believe that they are but the plain and unequivocal expressions of an opinion which I have always understood that Darwin held. And if any one has been led to think otherwise, I throw back this charge of "strained interpretation," by challenging such a person to adduce a single quotation from any part of Darwin's works, which can possibly be held to indicate that he regarded passages like those above quoted as in any way out of conformity with his theory of natural selection—or as put forward merely to "admit the possibility of explanations, to which really, however, he did not attach much importance." To the best of my judgement it is only some bias in favour of Mr. Wallace's views that can lead a naturalist to view in this way the clear and consistent expression of Darwin's.
That Mr. Wallace himself should be biassed in this matter might, perhaps, be expected. After rendering the following very unequivocal passage from the Origin of Species (p. 72)—"There can be little doubt that the tendency to vary in the same manner has often been so strong, that all individuals of the same species have been similarly modified without the aid of any form of selection"—Mr. Wallace says, "But no proof whatever is offered of this statement, and it is so entirely opposed to all we know of the facts of variation as given by Darwin himself, that the important word 'all' is probably an oversight." But, if Mr. Wallace had read the very next sentence he would have seen that here the important word "all" could not possibly have been "an oversight." For the passage continues,—"Or only a third, fifth, or tenth part of the individuals may have been thus affected, of which fact several instances could be given. Thus Graba estimates that about one-fifth of the guillemots in the Faroe Islands consist of a variety so well marked, that it was formerly ranked as a distinct species under the name of Uria lacrymans." And even if this passage had not been thus specially concerned with the question of the proportion in which "individuals of the same species have been similarly modified without the aid of any form of selection" the oversight with respect to "the important word 'all'" would still have remained an oversight of a recurrent character, as the following additional quotations from other parts of Darwin's writings may perhaps render apparent.
"There must be some efficient cause for each slight individual difference, as well as for more strongly marked variations which occasionally arise; and if the unknown cause were to act persistently, it is almost certain that all the individuals of the species would be similarly modified[168]."
"The acquisition of a useless part can hardly be said to raise an organism in the natural scale.... We are so ignorant of the exciting cause of the above specified modifications; but if the unknown cause were to act almost uniformly for a length of time, we may infer that the result would be almost uniform; and in this case all the individuals of the species would be modified in the same manner[169]."
Moreover, when dealing even with such comparatively slight changes as occur between our domesticated varieties—and which, a fortiori, are less likely to become "stable" through the uniform operation of causes other than selection, seeing that they are not only smaller in amount than occurs among natural species, but also have had but a comparatively short time in which to accumulate—Darwin is emphatic in his assertion of the same principles. For instance, in the twenty-third chapter of the Variation of Plants and Animals under Domestication, he repeatedly uses the term "definite action of external conditions," and begins the chapter by explaining his use of the term thus:—
"By the term definite action, as used in this chapter, I mean an action of such a nature that, when many individuals of the same variety are exposed during several generations to any change in their physical conditions of life, all, or nearly all, the individuals are modified in the same manner. A new sub-variety would thus be produced without the aid of selection[170]."
As an example of the special instances that he gives, I may quote the following from the same work:—
"Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same cause were to act uniformly during a long series of generations on many individuals, all probably would be modified in the same manner."
And, as instances of his more general statements in Chapter XXIII, these may suffice:—
"The direct action of the conditions of life, whether leading to definite or indefinite results, is a totally distinct consideration from the effects of natural selection.... The direct and definite action of changed conditions, in contradistinction to the accumulation of indefinite variations, seems to me so important that I will give a large additional body of miscellaneous facts[171]."