Chap. IX.] SUMMARY. 47 



power of absorbing each other by repeated crosses, and 

 in their inheritance of characters from both parent- 

 forms. Finally, then, although Ve are as ignorant of 

 the precise cause of the sterility of first crosses and of 

 hybrids as we are why animals and plants removed 

 from their natural conditions become sterile, yet the 

 facts given in this chapter do not seem to me opposed 

 to the belief that species aboriginally existed as varie- 

 ties. 



