probably developed? Truly, I think that the opponents of modification
and variation had better leave the argument 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 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! 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