proportionately; or to a much less extent, they differ in shape because the stimulation received by the structures is disproportionate.
Continuing the above line of argument, it follows that specific evolution in higher animals as regards size without change of shape, is due mainly to an evolution of the power to vary in response to stimulation proportionately in all the structures. Specific evolution as regards shape, whereby change of form is brought about, is mainly due to a disproportionate evolution in the structures of the power to vary in response to stimulation; thus as regards the giraffe, Natural Selection has resulted in the response to stimulation being greater in the structures of the fore than of the hind limbs, hence their greater size; whereas the reverse is the case as regards the kangaroo. In either case, notwithstanding the unequal evolution, there is perfect co-adaption of the parts—a co-adaption due in the individual to the strain which co-ordinated structures put on one another, whereby they are caused to develop proportionately; due in the species to the survival of those that possessed the power of varying co-adaptively, and the elimination of those which did not possess that power. In the giraffe the structures are much the same as in the kangaroo, that is, for each structure in the giraffe there is, generally speaking, a corresponding structure in the kangaroo, but the fully-developed animals differ vastly—differ not only as regards structural characteristics which are inborn, but also as regards structural characteristics which are acquired in the individual through stimulation acting during the ontogeny on the immense inborn power to vary in response along certain lines, the lines being different in the giraffe from what they are in the kangaroo.
It appears to me that the above considerations com-
