Chap. IX.] SUMMARY. 47 



there is the closest general resemblance between hybrids 

 and mongrels, — in their variability, in their 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 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. 



