18 HEREDITY AND SEX 



plasm must have changed. We have then the alter- 

 natives. Is there some internal, initial or driving im- 

 pulse that has led to the process of evolution ? Or has 

 the environment brought about changes in the germ- 

 plasm ? We can only reply that the assumption of an 





,W 







Fig. 5. — Schematic representation of the processes occurring during 

 the fertilization and subsequent segmentation of the ovum. (Boveri, from 

 Howell.) 



internal force puts the problem beyond the field of 

 scientific explanation. On the other hand, there is a 

 small amount of evidence, very incomplete and in- 

 sufficient at present, to show that changes in the en- 

 vironment reach through the soma and modify the 

 germinal material. 



