being forthcoming for many years to come. Experiments in
the hybridisation of animals are so difficult and tedious that even
Darwin never undertook any, and the only people who could and ought to
have done it--the Zoological Society--will not. There is one point,
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

_Frith Hill, Godalming. August 26, 1888._

Dear Mr. Hemsley,--You are aware that Patrick Geddes proposes to exclude
Natural Selection in the origination of thorns and spines, which he
imputes to "diminishing vegetativeness" or "ebbing vitality of the
species." It has occurred to me that insular floras should afford a test
of the correctness of this view, since in the absence of mammalia the
protection of spines would be less needed.

Your study of these floras will no doubt enable you to answer a few
questions on this point. Spines and thorns are, I believe, usually
abundant in arid regions of continents, especially in South