338 EVOLUTION, OLD AND NEW, 



which underlies variation generally, and this prin- 

 ciple is the power which all living beings possess 

 of slightly varying their actions in accordance with 

 varying needs, coupled with the fact observable through- 

 out nature that use develops, and disuse enfeebles an 

 organ, and that the effects, whether of use or disuse, 

 become hereditary after many generations. 



This resolves itself into the effect of the mutual 

 interaction of mind on body and of body on mind. 

 Thus he writes : — 



" The physical and the mental are to start with un- 

 doubtedly one and the same thing; this fact is most 

 easily made apparent through study of the organization 

 of the various orders of known animals. From the com- 

 mon source there proceeded certain effects, and these 

 effects, in the outset hardly separated, have in the 

 course of time become so perfectly distinct, that when 

 looked at in their extremest development they appear 

 to have little or nothing in common. 



"The effect of the body upon the mind has been 

 already sufficiently recognized; not so that of the mind 

 upon the body itself. The two, one in the outset though 

 they were, interact upon each other more and more 

 the more they present the appearance of having become 

 widely sundered, and it can be shown that each is 

 continually modifying the other and causing it to 

 vary."* 



And again, later : — 



" I shall show that the habits by which we now 



* ' PhlloBophie Zoologique,' ed. M. Martins, Paris, L;ons, 1873, 

 torn. 1. p. 24. 



