Here, then, we have in brief compass the whole essence of our opponents' argument. It is confessedly an argument _a priori_, a deduction from the theory of natural selection, a supposed consequence of that theory which is alleged to be so necessary that to dispute the consequence is tantamount to denying the theory from which it is derived. In short, as before stated, it is a question of theory, not a question of fact: our difference of opinion is logical, not biological: it depends on our interpretation of principles, not on our observation of species. It will therefore be my endeavour to show that the reasoning in question is fallacious: that it is _not_ a necessary deduction from the theory of natural selection that no characteristic form or marking, no peculiarities of instinct or of habit, can exist, but which must now be, or once have been, useful, or correlated with some other peculiarity that is useful. "The tuft of hair on the breast of a wild turkey-cock _cannot be of any use_, and it is doubtful whether it can be ornamental in the eyes of the female bird;--indeed, had the tuft appeared under domestication, it would have been called a monstrosity[90]." [90] _Origin of Species_, p. 70: italics mine. As a matter of common sense, unprejudiced by dogma, this appears to be a perfectly sound judgement; but if Wallace had asked Darwin to prove such a negative, Darwin could only have replied that it was for Wallace to prove the affirmative--and thus the issue would have been thrown back upon a discussion of general principles. Then Wallace would have said--"The assertion of inutility in the case of any organ or peculiarity which is not a rudiment or a correlation _is not, and can never be_, the statement of a fact, but _merely an expression of our ignorance of its purpose or origin_[91]." Darwin, however, would have replied:--"Our ignorance of the laws of variation is profound"; and while, on this account, we ought "to be extremely cautious in _pretending to decide what structures are now, or have formerly been, of use to each species_," in point of fact "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_[92]." [91] _Darwinism_, p. 137: italics mine. [92] _Origin of Species_, p. 72: Mr. Wallace himself quotes this passage (_Darwinism_, p. 141); but says with regard to it "the important word 'all' is probably an oversight." In the Appendix (II), on Darwin's views touching the doctrine of utility I adduce a number of precisely equivalent passages, derived from all his different works on evolution, and _every one of them_ presenting "the important word 'all.'" It will be my endeavour in the following discussion to show that Darwin would have had an immeasurable advantage in this imaginary debate.