XI PHENOMENA OF ORGANIC NATURE 465 
duced from a common stock two breeds which are 
not more or less fertile with one another. 
I do not know that there is a single fact which 
would justify any one in saying that any degree of 
sterility has been observed between breeds abso- 
lutely known to have been produced by selective 
breeding from a common stock. On the other 
hand, I do not know that there is a single fact 
which can justify any one in asserting that such 
sterility cannot be produced by proper experiment- 
ation. For my own part, I see every reason to 
believe that it may, and will beso produced. For, 
as Mr. Darwin has very properly urged, when we 
consider the phenomena of sterility, we find they 
are most capricious; we do not know what it is 
that the sterility depends on. There are some 
animals which will not breed in captivity ; whether 
it arises from the simple fact of their being shut 
up and deprived of their liberty, or not, we do not 
know, but they certainly will not breed. Whatan 
astounding thing this is, to find one of the most 
important of all functions annihilated by mere 
imprisonment ! 
So, again, there are cases known of animals 
which have been thought by naturalists to be un- 
doubted species, which have yielded perfectly fer- 
tile hybrids ; while there are other species which 
present what everybody believes to be varieties! 
1 And as I conceive with very good reason ; but if any objec- 
tor urges that we cannot prove that they have been produced by 
VOL. II H H 
