Page:The Effect of External Influences upon Development.djvu/22

18 as well as upon ontogeny, even though its results are not transmitted.

Let us take the well-known instance of the gradual increase in development of the deer's antlers, in consequence of which the head, in the course of generations, has become more and more heavily loaded. The question has been asked as to how it is possible for the parts of the body which have to support and move this weight to vary simultaneously and harmoniously if there is no such thing as the transmission of the effects of use or disuse, and if the changes have resulted from processes of selection only? This is the question put by Herbert Spencer as to &lsquo;co-adaptation,&rsquo; and the answer is to be found in connexion with the process of intra-selection. It is by no means necessary that all the parts concerned—skull, muscles and ligaments of the neck, cervical vertebrae, bones of the fore-limbs, &c.—should simultaneously adapt themselves by variation of the germ to the increase in size of the antlers; for in each separate individual the necessary adaptation will be temporarily accomplished by intra-selection—by the struggle of parts—under the trophic influence of functional stimulus.

The improvement of the parts in question, when so acquired, will certainly not be transmitted, but yet the primary variation is not lost. Thus when an advantageous increase in the size of the antlers has taken place, it does not lead to the destruction of the animal in consequence of the other parts being unable to suit themselves to it. All parts of the organism are in a certain degree variable and capable of being