species when crossed one with
another. It matters not one whit whether this sterility is universal,
or whether it exists only in a single case. Every hypothesis is bound
to explain, or, at any rate, not be inconsistent with, the whole of the
facts which it professes to account for; and if there is a single one of
these facts which can be shown to be inconsistent with (I do not merely
mean inexplicable by, but contrary to) the hypothesis, the hypothesis
falls to the ground,--it is worth nothing. One fact with which it is
positively inconsistent is worth as much, and as powerful in negativing
the hypothesis, as five hundred. If I am right in thus defining the
obligations of an hypothesis, Mr. Darwin, in order to place his
views beyond the reach of all possible assault, ought to be able to
demonstrate the possibility of developing from a particular stock by
selective breeding, two forms, which should either be unable to cross
one with another, or whose cross-bred offspring should be infertile with
one another.

For, you see, if you have not done that you have not strictly fulfilled
all the conditions of the problem; you have not shown that you can
produce, by the cause assumed, all the phenomena which you have in
nature. Here are the phenomena of Hybridism staring you in the face, and
you cannot say, 'I can, by selective modification, produce these same
results.' Now, it is admitted on all hands that, at present, so far as
experiments have gone, it has not been found possible to produce this
complete physiological divergence by selective breeding. I stated this
very clearly before, and I now refer to the point, because, if it could
be proved, not only that this HAS not been done, but that it CANNOT
be done; if it could be demonstrated that it is impossible to breed
selectively, from any stock, a form which shall not breed with another,
produced from the same stock; and if we were shown that this must be
the necessary and inevitable results of all experiments, I hold that Mr.
Darwin's hypothesis would be utterly shattered.

But has this been done? or what is really the state of the case? It is
simply that, so far as we have gone yet with our breeding, we have
not produced 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
absolutely known to have