244 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



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 conditions become sterile, yet the 

 facts given in this chapter do not seem to me opposed to the belief that 

 species aboriginally existed as varieties. 



