this is impossible in the case of inherited characters which are also _adaptive_ characters. How then does it fare with the case of inherited characters which are not also adaptive? Merely that this case is met by another and sequent assumption, which constitutes an integral part of the Neo-Darwinian creed--namely, that in nature there _can be no such characters_. Seeing that natural selection is taken to be the only possible cause of change in species, it follows that all changes occurring in species must necessarily be adaptive, whether or not we are able to perceive the adaptations. In this way apparently useless characters, as well as obviously useful ones, are ruled out of the question: that is to say, _all_ hereditary characters of species in a state of nature are _assumed_ to be due to natural selection, and then it is demanded that the validity of this assumption should be disproved by anybody who doubts it. Yet Weismann himself would be unable to suggest any conceivable method by which it can be disproved among species in a state of nature--and this even supposing that the assumption is entirely false[30]. [30] In subsequent chapters, especially devoted to the question (i.e. Section II), the validity of this assumption will be considered on its own merits. Consequently, the only way in which these speciously-sounding challenges can be adequately met is by removing some individuals of a species from a state of nature, and so from all known influences of natural selection; then, while carefully avoiding artificial selection, causing these individuals and their progeny through many generations unduly to exercise some parts of their bodies, or unduly to fail in the exercise of others. But, clearly, such an experiment is one that must take years to perform, and therefore it is now too early in the day to reproach the followers of Darwin with not having met the challenges which are thrown down by the followers of Weismann[31]. [31] I say "the followers of Weismann," because Weismann himself, with his clear perception of the requirements of experimental research, expressly states the above considerations, with the conclusions to which they lead. Nevertheless, he is not consistent in his utterances upon this matter; for he frequently expresses himself to the effect, "that the _onus probandi_ rests with my opponents, and therefore they ought to bring forward actual proofs" (_Essays_, i. p. 390). But, as above shown, the _onus_ rests as much with him as with his opponents; while, even if his opponents are right, he elsewhere recognizes that they can bring "actual proofs" of the fact only as a result of experiments which must take many years to perform.