HEREDITY AND EVOLUTION 109 



ground for the belief that similar operations are 

 concerned in the production of more complex 

 characters. Those who hesitate to draw such a 

 conclusion may well reflect upon the remarkable 

 effects of the " internal secretions " of the en- 

 zymes and hormones, and upon the extreme sus- 

 ceptibility of the developing embryo to even very 

 slight chemical changes in the surrounding me- 

 dium. It is my belief that in the direction here i 

 indicated lie the greatest possibilities of future! 

 investigations upon the cell, and that in the union 

 of cytology and biochemistry hes our greatest j 

 hope of future advance. 



HEREDITY AND EVOLUTION 



Lastly, if we accept the working hypothesis 

 that the primordia of determination are chemical 

 in nature, how may we conceive them to be so 

 modified as to produce new characters? It seems 1 

 to me that this question may well be reversed ; for 

 the wonder is, not that the idioplasm changes, but 

 that it adheres so stubbornly to its type. It may 

 as well be admitted that both our cytological and 

 our chemical knowledge in this direction is prac- 

 tically nil. It is well, further, to speak a word 

 of caution at this point. We must not forget 

 that some of the most acute and thoughtful of 

 naturalists have in recent years expressed the 

 conviction that the ultimate control of develop- 

 ment is not to be sought in the physico-chemical 

 properties of the germ-cells, but in an indwelling 



