Mr. Poulton has also in his book supplied a _résumé_ of the arguments for and against the theory of sexual selection in general. Of course in nearly all respects this corresponds with the _résumé_ which is given in the foregoing pages; but I have left the latter as it was originally written, because all the critical part is reproduced _verbatim_ from a review of Mr. Wallace's _Darwinism_, of a date still earlier than that of Mr. Poulton's book--viz. _Contemporary Review_, August, 1889. _Concluding Remarks._ I will now conclude this chapter, and with it the present volume, by offering a few general remarks on what may be termed the philosophical relations of Darwinian doctrine to the facts of adaptation on the one hand, and to those of beauty on the other. Of course we are all aware that before the days of this doctrine the facts of adaptation in organic nature were taken to constitute the clearest possible evidence of special design, on account of the wonderful mechanisms which they everywhere displayed; while the facts of beauty were taken as constituting no less conclusive evidence of the quality of such special design as beneficent, not to say artistic. But now that the Darwinian doctrine appears to have explained scientifically the former class of facts by its theory of natural selection, and the latter class of facts by its theory of sexual selection, we may fitly conclude this brief exposition of the doctrine as a whole by considering what influence such naturalistic explanations may fairly be taken to exercise upon the older, or super-naturalistic, interpretations. To begin with the facts of adaptation, we must first of all observe that the Darwinian doctrine is immediately concerned with these facts only in so far as they occur in organic nature. With the adaptations--if they can properly be so called--which occur in all the rest of nature, and which go to constitute the Cosmos as a whole so wondrous a spectacle of universal law and perfect order, this doctrine is but indirectly concerned. Nevertheless, it is of course fundamentally concerned with them to the extent that it seeks to bring the phenomena of organic nature into line with those of inorganic; and therefore to show that whatever view we may severally take as to the kind of causation which is energizing in the latter we must now extend to the former. This is usually expressed by saying that the theory of evolution by natural selection is a mechanical theory. It endeavours to comprise all the facts of adaptation in organic nature under the same category of explanation as those which occur in inorganic nature--that is to say, under the category of physical, or ascertainable, causation. Indeed, unless the theory has succeeded in doing this, it has not succeeded in doing anything--beyond making a great noise in the world. If Mr. Darwin has not discovered a new mechanical cause in the selection principle, his labour has been worse than in vain.