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. 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'.

*[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.

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.