In criticising Romanes, however, at the British Association, I want to
call your special attention to a point I have hardly made clear enough
in my paper. Romanes always speaks of the "physiological variety" as if
it were like any other _simple_ variety, and could as easily (he says
more easily) be increased. Whereas it is really complex, requiring a
remarkable correlation between different sets of individuals which he
never recognises. To illustrate what I mean, let me suppose a case. Let
there occur in a species three individual physiological varieties--A, B
and C--each being infertile with the bulk of the species, but quite
fertile with some small part of it. Let A, for example, be fertile with
X, Y and Z. Now I maintain it to be in the highest degree improbable
that B, a quite distinct individual, with distinct parents originating
in a distinct locality, and perhaps with a very different constitution,
merely because it also is sterile with the bulk of the species, should
be fertile with the very same individuals, X, Y, Z, that A is fertile
with. It seems to me to be at least 100 to 1 that it will be fertile
with some other quite distinct set of individuals. And so with C, and
any other similar variety. I express this by saying that each has its
"sexual complements," and that the complements of the one are almost
sure not to be the complements of the other. Hence it follows that A, B,
C, though differing in the same character of general infertility with
the bulk of the species, will really be three distinct varieties
physiologically, and can in no way unite to form a single physiological
variety. This enormous difficulty Romanes apparently never sees, but
argues as if all individuals that are infertile with the bulk of the
species must be or usually are fertile with the same set of individuals
or with each other. This I call a monstrous assumption, for which not a
particle of evidence exists. Take this in conjunction with my argument
from the severity of the struggle for existence and the extreme
improbability of the respective "sexual complements" coming together at