PERPETUATION OF LIVING BEINGS 87 



limitation. By selective breeding we can produce structural 

 divergences 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 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 overlooked, so that I could only 

 produce two of 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 1 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 can prove and this comes to the same thing 

 that varieties exist in nature within the limits of species, 

 and, what is more, that when a variety has come into 

 existence in nature, there are natural causes and conditions, 

 which are amply competent to pla# the part of a selective 

 breeder ; and although that is not quite the evidence that 

 one would like to have though it is not direct testimony 

 yet it is exceeding good and exceedingly powerful evidence 

 in its way. 



As to the first point, of varieties existing among natural 

 species, I might appeal to the universal experience of every 

 naturalist, and of any person who has ever turned any 

 attention at all to the characteristics of plants and animals 

 in a state of nature ; but I may as well take a few definite 

 cases, and I will begin with Man himself. 



I am one of those who believe that, at present, there 

 is no evidence whatever for saying, that mankind sprang 

 originally from any more than a single pair ; I must say, 

 that I cannot see any good ground whatever, or even any 



