116 CONDITIONS AFFECTING THE 



primitive type from which the animals have been pro- 

 bably developed ? Truly, I think that the opponents of 

 modification and variation had better leave the argu- 

 ment of recurrence alone, or it may prove altogether 

 too strong for them. 



To sum up, — the evidence as far as we have gone 

 is against the argument as to any limit to divergences, 

 so far as structure is concerned ; and in favour of 

 a physiological limitation. By selective breeding we 

 can produce structural divergences as great as those of 

 species, but we cannot produce equal physiological diver- 

 gences. 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 telling 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 obtaining evidence even of the 

 first origin of those varieties which we know to have 

 occurred in domesticated animals. I told you, that 

 almost always the origin of these varieties is over- 

 looked, 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 



