ORGANIC EVOLUTION — THE FACTORS 119 



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 stimula- 

 tion 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 gu-affe 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 regai'ds 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 

 kangai'oo. 



It appears to me that the above considerations com- 



