SUMMARY OF THE WORK 325 



including college professors on one hand and 

 a few practical breeders of plants and animals 

 on the other we must consider yet another 

 aspect of the intellectual atmosphere of the 

 closing decade of the nineteenth century. 



We must understand that in this period, 

 whereas the general doctrine of evolution had 

 been accepted, there was wide diversity of 

 opinion as to many of its important details. It 

 could scarcely be said that there was any preva- 

 lent doctrine as to what forces in nature caused 

 the observed variation between wild forms of 

 plant and animal life upon which the operation 

 of natural selection is based. 



The "survival of the fittest" was an accepted 

 doctrine, but the origin of the fittest was an 

 unsolved enigma. 



A suggestion that new forms might arise by 

 combining existing species had occurred, doubt- 

 less, to many minds. But this idea was com- 

 bated or annulled by the prevalent notion that 

 the offspring of hybrid species are necessarily 

 infertile. 



It is true that a few plant breeders, notably 

 Dean Herbert and Andrew Knight, had advo- 

 cated the idea that hybrids between true species 

 may be fertile, and had even seemed to demon- 

 strate the truth of this view some three genera- 



