~ ee ee | 
eel Goma 
XI PHENOMENA OF ORGANIC NATURE 499 
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 
oe 
_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 
