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 experimentation. For my own part, I see every
reason to believe that it may, and will be so 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. What an 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 undoubted species, which have yielded perfectly
fertile hybrids; while there are other species which present what
everybody believes to be varieties* which are more or less infertile
with one another. ([Footnote] *And as I conceive with very good reason;
but if any objector urges that we cannot prove that they have been
produced by artificial or natural selection, the objection must be
admitted--ultrasceptical as it is. But in science, scepticism is a
duty.) There are other cases which are truly extraordinary; there is
one, for example, which has been carefully examined,--of two kinds of
sea-weed, of which the male element of the one, which we may call A,
fertilizes the female element of the other, B; while the male element of
B will not fertilize the female element of A; so that, while the former
experiment seems to show us that they are 'varieties', the latter leads
to the conviction that they are 'species'.

When we see how capricious and uncertain this sterility is, how unknown
the conditions on which it depends, I say that we have no right to
affirm that those conditions will not be better understood by and
by, and we have no ground for supposing that we may not be able to
experiment so as to obtain that crucial result which I mentioned
just now. So that though Mr. Darwin's hypothesis does not completely
extricate us from this difficulty at present, we have not the least
right to say it will not do so.

There is a wide gulf between the thing you cannot explain and the thing
that upsets you altogether. There is hardly any h