120 ORGANIC EVOLUTION — THE FACTORS 



pletely dispose of Mr. Spencer's objections. The co- 

 adaption of animal parts is clearly due to, and maintained 

 by, the inborn power the parts possess of developing in 

 response to stimulation, so that the development or 

 atrophy of any part, owing to increase or decrease of 

 stimulation, is followed, owing to alterations in the 

 strain put upon them, and therefore also to increase 

 or decrease of stimulation, by the development or 

 atrophy of all co-ordinated parts. It must be remem- 

 bered that Mr. Spencer, while admitting that inborn 

 variations are transmissible, attributes co-adaptive evo- 

 lution to the accumulated effects of use or disuse, that 

 is, he attributes it to the accumulation of those varia- 

 tions Avhich are acquired in response to stimulation. 

 But in the offspring these variations are not reproduced 

 except in response to fresh stimulation, therefore, as 

 regards any structure, that which is inherited is not the 

 variation, but only the power to acquire the variation 

 afresh in response to stimulation. Now there is no 

 tittle of evidence tending to prove that stimulation 

 increases the 2J0ivcr of a structure to vary. The power 

 to vary is inherent ; it is called into activity by stimu- 

 lation, but there is nothing to show that the inherent 

 ability to vary is increased by stimulation. On the 

 contrary, as animals grow older, their structures become 

 less and less responsive to stimulation ; the power of 

 responding to stimulation in their long-stimulated 

 structures actually declines, and the structures atrophy. 

 Therefore as the power of varying is not increased or 

 (presumably) decreased by stimulation or the lack of it, 

 by use or disuse, it is clear, since variations of this power 

 cannot be acquired, that acquired variations in it cannot 

 be transmitted, and therefore if we hold that acquired 

 variations are transmissible, we must not attribute the 

 reproduction in the child of the parent's acquired vari- 

 ations to transmitted increase or decrease of this power. 



