Thus I have no doubt that Mr. Darwin's hypothesis will be found
competent to explain the majority of the phenomena exhibited by species
in nature; but in an earlier lecture I spoke cautiously with respect to
its power of explaining all the physiological peculiarities of species.

There is, in fact, one set of these peculiarities which the theory of
selective modification, as it stands at present, is not wholly
competent to explain, and that is the group of phenomena which I
mentioned to you under the name of Hybridism, and which I explained to
consist in the sterility of the offspring of certain 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,