I heard yesterday to my joy that Dr. Hildebrand has been experimenting on the direct action of pollen on the mother-plant with success. He has also succeeded in making a true graft-hybrid between two varieties of potatoes, in which I failed. I look at this as splendid for pangenesis, as being strong evidence that bud-reproduction and seminal reproduction do not essentially differ.
My book is horribly delayed, owing to the accursed index-maker. (207/2. Darwin thoroughly appreciated the good work put into the index of "The Variation of Animals and Plants.") I have almost forgotten it!
LETTER 208. TO T.H. HUXLEY. Down, January 30th .
Most sincere thanks for your kind congratulations. I never received a note from you in my life without pleasure; but whether this will be so after you have read pangenesis (208/1. In Volume II. of "Animals and Plants, 1868.), I am very doubtful. Oh Lord, what a blowing up I may receive! I write now partly to say that you must not think of looking at my book till the summer, when I hope you will read pangenesis, for I care for your opinion on such a subject more than for that of any other man in Europe. You are so terribly sharp-sighted and so confoundedly honest! But to the day of my death I will always maintain that you have been too sharp-sighted on hybridism; and the chapter on the subject in my book I should like you to read: not that, as I fear, it will produce any good effect, and be hanged to you.
I rejoice that your children are all pretty well. Give Mrs. Huxley the enclosed (208/2. Queries on Expression.), and ask her to look out when one of her children is struggling and just going to burst out crying. A dear young lady near here plagued a very young child for my sake, till it cried, and saw the eyebrows for a second or two beautifully oblique, just before the torrent of tears began.
The sympathy of all our friends about George's success (it is the young Herald) (208/3. His son George was Second Wrangler in 1868; as a boy he was an enthusiast in heraldry.) has been a wonderful pleasure to us. George has not slaved himself, which makes his success the more satisfactory. Farewell, my dear Huxley, and do not kill yourself with work.
(209/1. The following group of letters deals with the problem of the causes of the sterility of hybrids. Mr. Darwin's final view is given in the "Origin," sixth edition (page 384, edition 1900). He acknowledges that it would be advantageous to two incipient species, if by physiological isolation due to mutual sterility, they could be kept from blending: but he continues, "After mature reflection it seems to me that this could not have been effected through Natural Selection." And finally he concludes (page 386):--
"But it would be superfluous to discuss this question in detail; for with plants we have conclusive evidence that the sterility of crossed species must be due to some principle quite independent of Natural Selection. Both Gartner and Kolreuter have proved that in genera including numerous species, a series can be formed from species which when crossed yield fewer and fewer seeds, to species which never produce a single seed, but yet are affected by the pollen of certain other species, for the germen swells. It is here manifestly impossible to select the more sterile individuals, which have already ceased to yield seeds; so that this acme of sterility, when the germen alone is affected, cannot have been gained through selection; and from the laws governing the various grades of sterility being so uniform throughout the animal and vegetable kingdoms, we may infer that the cause, whatever it may be, is the same or nearly the same in all cases."