Again, Weismann's theory, that the somatic and the germ-plasm determinants may be similarly and simultaneously modified by external conditions may be extended much further than he has used it himself, so as to exclude, or at any rate invalidate, _all_ evidence in favour of Lamarckianism, other than the inheritance of the effects of use and disuse. All evidence from apparently inherited effects produced by change of external conditions is thus virtually put out of court, leaving only evidence from the apparently inherited effects of functionally produced modifications. And this line of evidence is invalidated by Panmixia. Hence there remain only the arguments from selective value and co-adaptation. Weismann meets these by adducing the case of neuter insects, which have been already considered at sufficient length. (C.) _Experimental Evidence as to the Non-inheritance of Acquired Characters._ Let us now proceed to the experimental evidence which has been adduced on the side of Weismannism. Taking this evidence in order of date, we have first to mention that on which the school of Weismann has hitherto been satisfied almost exclusively to rely. This is the line of negative evidence, or the seeming absence of any experimental demonstration of the inheritance of acquired characters. This kind of evidence, however, presents much less cogency than is usually supposed. And it has been shown in the last chapter that the amount of experimental evidence in favour of the transmission of acquired characters is more considerable than the school of Weismann seems to be aware--especially in the vegetable kingdom. I do not think that this negative line of evidence presents much weight; and, to show that I am not biassed in forming this judgement, I may here state that few have more reason than myself for appreciating the weight of such evidence. For, as already stated, when first led to doubt the Lamarckian factors, now more than twenty years ago, I undertook a research upon the whole question--only a part of which was devoted to testing the particular case of Brown-Séquard's statements, with the result recorded in the preceding chapter. As this research yielded negative results in all its divisions--and, not only in the matter of Brown-Séquard's statements--I have not hitherto published a word upon the subject. But it now seems worth while to do so, and for the following reasons.