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 Africa, where large herbivorous mammals abound.  Now, if the long-continued presence of these mammals is a factor in the production of spines by Natural Selection, they should be wholly or comparatively absent in regions equally arid where there are no mammals.  The Galapagos seem to be such a case—­also perhaps some of the Sandwich Islands, and generally the extra-tropical volcanic islands.  Also Australia comparatively, and the highlands of Madagascar.

Of course, the endemic species must be chiefly considered, as they have had time to be modified by the conditions.  If you can give me the facts, or your general impression from your study of these floras, I shall be much obliged.  I see, of course, many other objections to Geddes’s theory, but this seems to offer a crucial test.—­Believe me yours very truly,