Jermyn Street, April 30, 1863.
My dear Kingsley,
I am exceedingly pleased to have your good word about the lectures,—and I think I shall thereby be encouraged to do what a great many people have wished—that is, to bring out an enlarged and revised edition of them.
The only difficulty is time—if one could but work five-and-twenty hours a day!
With respect to the sterility question, I do not think there is much doubt as to the effect of breeding in and in in destroying fertility. But the sterility which must be obtained by the selective breeder in order to convert his morphological species into physiological species—such as we have in nature—must be quite irrespective of breeding in and in.
There is no question of breeding in and in between a horse and an ass, and yet their produce is usually a sterile hybrid.
So if Carrier and Tumbler, e.g., were physiological species equivalent to Horse and Ass, their progeny ought to be sterile or semi-sterile. So far as experience has gone, on the contrary, it is perfectly fertile—as fertile as the progeny of Carrier and Carrier or Tumbler and Tumbler.
From the first time that I wrote about Darwin's book in the "Times" and in the "Westminster" until now, it has been obvious to me that this is the weak point of Darwin's doctrine. He HAS shown that selective breeding is a vera causa for morphological species; he has not yet shown it a vera causa for physiological species.
But I entertain little doubt that a carefully devised system of experimentation would produce physiological species by selection—only the feat has not been performed yet.
I hope you received a copy of "Man's Place in Nature," which I desired should be sent to you long ago. Don't suppose I ever expect an acknowledgment of the book—it is one of the greatest nuisances in the world to have that to do, and I never do it—but as you mentioned the Lectures and not the other, I thought it might not have reached you. If it has not, pray let me know and a copy shall be forwarded, as I want you very much to read Essay Number 2.