however, I think you have overlooked. You urge the improbability of the
required infertility being correlated with the particular variations
which characterised each incipient species. But the whole point of my
argument is, that the physiological adjustments producing fertility are
so delicate that they are disturbed by almost any variation or change of
conditions--except in the case of domestic animals, which have been
domesticated because they are not subject to this disturbance. The whole
first half of the chapter is to bring out this fact, which Darwin has
dwelt upon, and it certainly does afford a foundation for the assumption
that usually, and in some considerable number of individuals, variation
in nature, accompanied by somewhat changed conditions of life, is
accompanied by, and probably correlated with, some amount of
infertility. No doubt this assumption wants proving, but in the meantime
I am glad you think that, granting the assumption, I have shown that
Natural Selection is able to accumulate sterility variations.

That is certainly a step in advance, and we cannot expect to do more
than take very short theoretical steps till we get more facts to rest
upon. If you should happen to come across any facts which seem to bear
upon it, pray let me know. I can find none but those I have referred to.

I have just finished a chapter on male ornament and display, which I
trust will help to clear up that point--Believe me yours very
faithfully,

ALFRED R. WALLACE.

* * * * *

TO DR. W.B. HEMSLEY