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
the right time, and I think Romanes' ponderous paper is disposed of.

I wrote my paper, however, quite as much to expose the great presumption
and ignorance of Romanes in declaring that Natural Selection is _not_ a
theory of the origin of species--as it is calculated to do much harm.
See, for instance, the way the Duke of Argyll jumped at it like a trout
at a fly!--Yours very faithfully,

ALFRED R. WALLACE.

* * * * *

The earlier part of the next letter refers to "The Experimental Proof of
the Protective Value of Colour and Markings in Insects in reference to
their Vertebrate Enemies," in the _Proceedings of the Zoological Society
of London_, 1887, p. 191.

TO PROF. POULTON

_Frith Hill, Godalming. October 20, 1887._

My dear Poulton,--It is very interesting to me to see how very generally
the facts are in accordance with theory, and I am only surprised that
the exceptions and irregularities are not more numerous than they are
found to be. The only difficult case, that of _D. euphorbiae_, is due
probably to incomplete knowledge. Are lizards and sea-birds the only, or
even the chief, possible enemies of the species? They evidently do not
prevent its coming to maturity in considerable abundance, and it is
therefore no doubt preserved from its chief enemies during its various
stages of growth.

The only point on which I differ from you--as you know--is your
acceptance, as proved, of the theory of sexual c