46 The Direction of Evolution 



variations are to be perpetuated and accumulated can be 

 answered only by undertaking an investigation of this 

 correlation ; that is, by interpreting the actual accommoda- 

 tions, intelligent and other, which the individuals make. 

 The use made of the plasticity by the intelligence, there- 

 fore, becomes the critically important thing for evolution 

 theory, even though it assumes the presence of the plas- 

 ticity itself. 



It may be said, indeed, quite truly, that this value of 

 accommodation is implicit in the theory of natural selec- 

 tion; for, according to that theory, there is continued selec- 

 tion of certain fit individuals, and their fitness may consist 

 in their being plastic or ' accommodating.' This is es- 

 pecially true of the theory of Roux, which makes use of 

 what he calls 'the struggle of the parts,' and of Weis- 

 mann's ' intra-selection ' theory. Yet still the qualifica- 

 tions of the fit individuals are not given in their plasticity, 

 but they arise only in the course of development ; and they 

 may take on many different forms. There may be alterna- 

 tive ways in which the same plastic material or organism 

 may adjust itself to the conditions of life. The same 

 emergency may lead animals of common heredity and 

 equal plasticity to make vital adjustments so different in 

 kind that each may start a new line of evolutionary prog- 

 ress. In fact, I think many cases of divergent evolution 

 have actually begun in such a situation (cf. Chap. XIII. 

 2, 3). How, then, is it possible to say that both these 

 differing lines of descent are equally accounted for by the 

 same degree of plasticity in the individuals who are their 

 common progenitors ? 



Suppose two creatures born with the same degree of 

 plasticity in respect to a certain function, but with differ- 



