XI PHENOMENA OF ORGANIC NATURE 429 



in favour of a physiological limitation. By selec 

 tive breeding we can produce structural diver 

 gences as great as those of species, but we cannot 

 produce equal physiological divergences. For the 

 present I leave the question there. 



Now, the next problem that lies before us and 

 it is an extremely important one is this : Does 

 this selective breeding occur in nature ? Because, 

 if there is no proof of it, all that I have been tell 

 ing you goes for nothing in accounting for the 

 origin of species. Are natural causes competent 

 to play the part of selection in perpetuating 

 varieties ? Here we labour under very great 

 difficulties. In the last lecture I had occasion to 

 point out to you the extreme difficulty of obtain 

 ing evidence even of the first origin of those 

 varieties which we know to have occurred in 

 domesticated animals. I told you, that almost al 

 ways the origin of these varieties is overlooked, so 

 that I could only produce two or three cases, as 

 that of Gratio Kelleia and of the Ancon sheep. 

 People forget, or do not take notice of them until 

 they come to have a prominence ; and if that is 

 true of artificial cases, under our own eyes, and in 

 animals in our own care, how much more difficult 

 it must be to have at first hand good evidence of 

 the origin of varieties in nature ! Indeed, I do 

 not know that it is possible by direct evidence to 

 prove the origin of a variety in nature, or to prove 

 selective breeding ; but I will tell you what we 



