On problem of sterility, CD cannot persuade himself that it has been gained by natural selection.
On sexual selection and minute variations, he tends to agree with ARW. Sends George Darwin's notes on ARW's argument.
4 Chester Pl<ace> | R. Park | N.<W>
My dear Wallace
I have sent your query to Cambridge to my son. He ought to
answer it, for he got his place of 2
I have told E. Edwards to send <one> of my large Photographs to
you addressed to 76
The sterility is a most a<wkward> problem. I can see so far <but I> am hardly willing to admit all your assumptions & even if they were all admitted, the process is so complex, & the sterility (as you remark in your note) so universal, even with species inhabiting quite distinct countries (as I remark in my Chapter), together with the frequency of a difference in reciprocal unions, that I cannot persuade myself that it has been gained by natural selection, any more than the difficulty of grafting distinct genera, & the impossibility of grafting distinct Families.
You will allow, I suppose, that the capacity of grafting has not been directly acquired through natural selection.—
<I think> that you will be pleased <with> the 2
In regard to sexual selection. A girl sees a handsome man, & without observing whether his nose or whiskers are the tenth of an inch longer or shorter than in some other man, admires his appearance & says she will marry him. So I suppose with the pea-hen; & the tail has been increased in length merely by on the whole presenting a more gorgeous appearance. J. Jenner Weir, however, has given me some facts showing that birds apparently admire details of plumage.—
Yours most sincerely | C. Darwin
``The result is, that in this area hybrids will not increase so rapidly as before.''
The fact, that the two forms of the Species have become slightly sterile together, implies that in this ``certain definite area'' after some period after the epoch when the observations are supposed to begin the total number of individuals is less than it was initially. At this period these individuals consist of the two pure forms and a certain number of hybrids. Outside the ``area'' the number of individuals will be the same as at first, and the population will consist of the two pure forms and hybrids.—
It is in the number of these hybrids that the decrease of
population in the ``area'' arises, and that outside the area
the population is kept up.— Thus inside the area there will
be a deficiency, which may and certainly will be made up; this
may take place in one of two ways—or in a combination of
them—viz (1) by invasion from the outside (2) by greater
numbers of the families of the pure forms surviving, thro'
being less crowded. Now as the Struggle will take place between
the young animals inside the area & old ones outside—it
appears almost certain that invasion will take place. Thus if
we suppose all the animals in the ``area'' to crowd together to
yield to the external pressure—the area (the space occupied
by the physiological variety) may be said to decrease in size.
According to M
It is assumed that there are no cross unions between A
Now after the first generation the offspring of the
∴ The offspring of the
- f1 6033.f1The date is established by the relationship between this letter and the letter from A. R. Wallace, 19 March 1868. In 1868, the first Saturday after 19 March was 21 March.
- f2 6033.f2CD refers to George Howard Darwin. See letter to G. H. Darwin, 24 January  and n. 2. For Wallace's query, see the letter from A. R. Wallace, 19 March 1868 and n. 6.
- f3 6033.f3See enclosure. George commented on two of the points enumerated by Wallace in the enclosure to Wallace's letter of 1 March 1868. For Wallace's response to George's comments, see the letter from A. R. Wallace, 24 March .
- f4 6033.f4See letter from A. R. Wallace, 19 March 1868 and n. 1. Westbourne Grove was Wallace's London address. He had recently been staying at Hurstpierpoint, the home of his wife's parents (see letter from A. R. Wallace, 19 March 1868).
- f5 6033.f5See letter from A. R. Wallace, 1 March 1868. CD refers to Variation 2: 185--6.
- f6 6033.f6CD refers to the second volume of the tenth edition of Charles Lyell's Principles of Geology (Lyell 1867--8).
- f7 6033.f7For Wallace's objection concerning sexual selection and minute variations, see the letter from A. R. Wallace, 19 March 1868.
- f8 6033.f8CD probably refers to John Jenner Weir's remarks in his letter of 16 March 1868.
- f9 6033.f9George is quoting from Wallace's note on sterility (see enclosure to letter from A. R. Wallace, 1 March 1868). The numbers heading the arguments refer to Wallace's numbering of points in his original note.