292 SUMMARY. 



power of absorbing each other by repeated crosses, and in 

 their inheritance of characters from both parent forms. 

 Finally, then, although we 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 con- 

 ditions become sterile, yet the facts given in this chapter do 

 not seem to me opposed to the belief that species aboriginally 

 existed as varieties. 



