First, then, for the reasons _a priori_--and they are confessedly _a priori_--which have been adduced to prove that natural selection has been what in Darwin's opinion it has not been,--"the _exclusive_ means of modification." Disregarding the Lamarckian factors--which, even if valid, have but little relation to the present question, seeing that they are concerned, almost exclusively, with the evolution of _adaptive_ characters--it is alleged that natural selection must occupy the whole field, because no other principle of change can be allowed to operate in the presence of natural selection. Now, I fully agree that this statement may hold as regards any principle of change which is deleterious; but clearly it does not hold as regards any principle which is merely neutral. If any one were to allege that specific characters are frequently detrimental to the species presenting them, he would no doubt lay himself open to the retort that natural selection could not allow such characters to persist; or, which amounts to the same thing, that it _does_ "necessarily follow from the theory of natural selection" that specific characters can never be in any large number, or in any large measure, _harmful_ to the species presenting them. But where the statement is that specific characters are frequently _indifferent_--again to use Professor Huxley's term--the retort loses all its relevancy. No reason has ever been shown why natural selection should interfere with merely indifferent characters, supposing such to have been produced by any of the agencies which we shall presently have to consider. Therefore this argument--or rather assertion--goes for nothing. The only other argument I have met with on this side of the question is one that has recently been adduced by Mr. Wallace. He says:-- "One very weighty objection to the theory that _specific_ characters can ever be wholly useless appears to have been overlooked by those who have maintained the frequency of such characters, and that is, their almost necessary instability[94]." [94] _Darwinism_, p. 138. This argument he proceeds to elaborate at considerable length, but fails to perceive what appears to me the obvious answer. Provided that the cause of the useless character is constant, there is no difficulty in understanding why the character is stable. Utility is not the only principle that can lead to stability: any other principle must do the same, provided that it acts for a sufficient length of time, and with a sufficient degree of uniformity, on all the individuals of a species. This is a consideration the cogency of which was clearly recognized by Darwin, as the following quotations will show. Speaking of unadaptive characters, he says they may arise as merely "fluctuating variations, which sooner or later become _constant_ through the nature of the organism and of surrounding conditions, _but not through natural selection_[95]."