My dear Wallace,—­You cannot well imagine how much I have been pleased by what you say about Pangenesis.  None of my friends will speak out, except, to a certain extent, Sir H. Holland,[67] who found it very tough reading, but admits that some view “closely akin to it” will have to be admitted.  Hooker, as far as I understand him, which I hardly do at present, seems to think that the hypothesis is little more than saying that organisms have such and such potentialities.  What you say exactly and fully expresses my feeling, viz. that it is a relief to have some feasible explanation of the various facts, which can be given up as soon as any better hypothesis is found.  It has certainly been an immense relief to my mind; for I have been stumbling over the subject for years, dimly seeing that some relation existed between the various classes of facts.  I now hear from H. Spencer that his views quoted in my footnote refer to something quite distinct, as you seem to have perceived.

I shall be very glad to hear, at some future day, your criticisms on the causes of variability.

Indeed, I feel sure that I am right about sterility and Natural Selection.  Two of my grown-up children who are acute reasoners have two or three times at intervals tried to prove me wrong, and when your letter came they had another try, but ended by coming back to my side.  I do not quite understand your case, and we think that a word or two is misplaced.  I wish some time you would consider the case under the following point of view.  If sterility is caused or accumulated through Natural Selection, then, as every degree exists up to absolute barrenness, Natural Selection must have the power of increasing it.  Now take two species, A and B, and assume that they are (by any means) half-sterile, i.e. produce half the full number of offspring.  Now try and make (by Natural Selection) A and B absolutely sterile when crossed, and you will find how difficult it is.  I grant, indeed it is certain, that the degree of sterility of the individuals of A and B will vary, but any such extra-sterile individuals of, we will say, A, if they should hereafter breed with other individuals of A, will bequeath no advantage to their progeny, by which these families will tend to increase in number over other families of A, which are not more sterile when crossed with B. But I do not know that I have made this any clearer than in the chapter in my book.  It is a most difficult bit of reasoning, which I have gone over and over again on paper with diagrams.

I shall be intensely curious to see your article in the Journal of Travel.

Many thanks for such answers as you could give.  From what you say I should have inferred that birds of paradise were probably polygamous.  But after all, perhaps it is not so important as I thought.  I have been going through the whole animal kingdom in reference to sexual selection, and I have just got to the beginning of Lepidoptera, i.e. to end of insects, and shall then pass on to Vertebrata.  But my ladies next week are going (ill-luck to it) to take me nolens-volens to London for a whole month.