Dear Mr. Henslow,--I am much indebted to you for all your most interesting letters, and also for prospect of receiving your books. Although forbidden to write letters myself, or to think about anything as yet, I must send a few lines, pending arrival of the books and papers, giving my general impression of your views as set out in your correspondence. Briefly, it seems to me that your argument is perfectly clear up to a certain point, but then suddenly becomes a _petitio principii_. In other words, so far as your view is critical of natural selection considered as a _hypothetical cause_ of adaptive evolution, I can well believe you have adduced a formidable array of facts. But I fail to follow, when you pass on to the constructive part of your case--or your suggested substitute for natural selection in self-adaptation. For self-adaptation, I understand, consists in results of _immediate response to stimuli supplied by environment_. But, if so, surely the statement that all the adaptive machinery of plant-organisation is due to self-adaptation is a mere begging of the question _against_ natural selection _unless it can be shown how self-adaptation works in each case_. Now I do not find any suggestion as to this. And yet this is obviously the essential point; since, _unless it can be shown how self-adaptation works_--_i.e._ that it is a _vera causa_, and not a mere word serving to re-state the facts of adaptive evolution. We have got no further in the way of _explanation_ than the physician, who said, that the reason why morphia produces sleep is because it possesses a soporific quality. Observe, I purposely abstain from considering your criticism of natural selection, which, although perfectly lucid and possibly justifiable, yet certainly does admit of the answer that incipient variations of a _fortuitous_ kind under nature _may_ often be inconspicuous (while Wallace shows that in animals they are, as a matter of fact, usually considerable). But we need not go into this. The interesting point to all of us must be the constructive part of your work; and I have tried to explain _my_ difficulty with regard to it. _Why should protoplasm be able to adapt itself_ into the millions of diverse mechanisms of nature by converse with environment? The theory of natural selection gives a logically possible, even if it be a biologically inadequate answer. But I cannot see that the theory of self-adaptation does, _unless it can be shown that there is some sufficient reason why_, say a _direct-environment should produce self-adaptation_ in the direction of hairs, a marine one in that of fleshiness, &c. &c.