natural selection is in large part presented with the raw material of its manufacture--special material of the particular kinds required, as distinguished from promiscuous material of all kinds. And the more complex the manufacture the more important will be the work of this subordinate factory. We can well imagine how the shell of a nut, for instance, or even the protective colouring of an insect, may have been gradually built up by natural selection alone. But just in proportion as structures or organs are not merely thus of passive _use_ (where, of course, the Lamarckian principles cannot obtain), but require to be actively _used_, in that proportion does it become difficult to understand the _incipient_ construction of them by natural selection alone. Therefore, in many such cases, if the incipient construction is not to be explained by the Lamarckian principles, it is difficult to see how it is to be explained at all. [85] E.g. "The supposed transmission of this artificially produced disease (epilepsy) is the only definite instance which has been brought forward in support of the transmission of acquired characters."--_Essays_, p. 328. Furthermore, since the question as to the transmission of acquired characters stands now exactly as it did after the publication of Mr. Galton's _Theory of Heredity_ twenty years ago, it would seem that our judgement with regard to it should remain exactly what it was then. Although we must "out-Darwin Darwin" to the extent of holding that he assigned too large a measure of intensity to the Lamarckian factors, no sufficient reason has been shown for denying the existence of these factors _in toto_; while, on the other hand, there are certain general considerations, and certain particular facts, which appear to render it probable that they have played a highly important part in the process of organic evolution as a whole. At the same time, and in the present state of our information, this judgement must be deemed provisional, or liable eventually to be overturned by experimental proof of the non-inheritance of acquired characters. But, even if this should ever be finally accomplished, the question would still remain whether the principle of natural selection alone is capable of explaining all the facts of adaptation; and, for my own part, I should then be disposed to believe that there must be some other, though hitherto undiscovered, principle at work, which co-operates with natural selection, by playing the subordinate role which was assigned by Darwin to the principles of Lamarck.