What I have called unconscious selection by man illustrates, as it seems to me, the same principle as yours, within the same area. Man purchases the individual animals or plants which seem to him the best in any respect--some more so, and some less so--and, without any matching or pairing, the breed in the course of time is surely altered. The absence in numerous instances of intermediate or blending forms, in the border country between two closely allied geographical races or close species, seemed to me a greater difficulty when I discussed the subject in the "Origin."
With respect to your illustration, it formerly drove me half mad to attempt to account for the increase or diminution of the productiveness of an organism; but I cannot call to mind where my difficulty lay. (279/2. See Letters 209-16.) Natural Selection always applies, as I think, to each individual and its offspring, such as its seeds, eggs, which are formed by the mother, and which are protected in various ways. (279/3. It was in regard to this point that Romanes had sent the MS. to Darwin. In a letter of June 16th he writes: "It was with reference to the possibility of Natural Selection acting on organic types as distinguished from individuals,--a possibility which you once told me did not seem at all clear.") There does not seem any difficulty in understanding how the productiveness of an organism might be increased; but it was, as far as I can remember, in reducing productiveness that I was most puzzled. But why I scribble about this I know not.
I have read your review of Mr. Allen's book (279/4. See "Nature" (June 7th, 1877, page 98), a review of Grant Allen's "Physiological Aesthetics."), and it makes me more doubtful, even, than I was before whether he has really thrown much light on the subject.
I am glad to hear that some physiologists take the same view as I did about your giving too much credit to H. Spencer--though, heaven knows, this is a rare fault. (279/5. The reference is to Romanes' lecture on Medusa, given at the Royal Institution, May 25th. (See "Nature," XVI., pages 231, 269, 289.) It appears from a letter of Romanes (June 6th) that it was the abstract in the "Times" that gave the impression referred to. References to Mr. Spencer's theories of nerve-genesis occur in "Nature," pages 232, 271, 289.)
The more I think of your medusa-nerve-work the more splendid it seems to me.
LETTER 280. TO A. DE CANDOLLE. Down, August 3rd, 1877.
I must have the pleasure of thanking you for your long and interesting letter. The cause and means of the transition from an hermaphrodite to a unisexual condition seems to me a very perplexing problem, and I shall be extremely glad to read your remarks on Smilax, whenever I receive the essay which you kindly say that you will send me. (280/1. "Monographiae Phanerogamarum," Volume I. In his treatment of the Smilaceae, De Candolle distinguishes:--Heterosmilax which has dioecious flowers without a trace of aborted stamens or pistils, Smilax with sterile stamens in the female flowers, and Rhipogonum with hermaphrodite flowers.) There is much justice in your criticisms (280/2. The passage criticised by De Candolle is in "Forms of Flowers" (page 7): "It is a natural inference that their corollas have been increased in size for this special purpose." De Candolle goes on to give an account of the "recherche linguistique," which, with characteristic fairness, he undertook to ascertain whether the word "purpose" differs in meaning from the corresponding French word "but.") on my use of the terms object, end, purpose; but those who believe that organs have been gradually modified for Natural Selection for a special purpose may, I think, use the above terms correctly, though no conscious being has intervened. I have found much difficulty in my occasional attempts to avoid these terms, but I might perhaps have always spoken of a beneficial or serviceable effect. My son Francis will be interested by hearing about Smilax. He has dispatched to you a copy of his paper on the glands of Dipsacus (280/3.