More on the "terrible problem" of natural selection and sterility. CD's reasons for disagreeing with ARW. CD analyses and answers ARW in detail in defence of his conclusion that sterility cannot be increased through natural selection.
Down Bromley Kent
My dear Wallace
I have been considering the terrible problem.— Let me first say
that no man could have more earnestly wished for the success of
N. selection in regard to sterility, than I did; & when I considered a
general statement, (as in your last note) I always felt sure it could
be worked out, but always failed in detail. The cause being as I
believe, that natural selection cannot effect what is not good for the
individual, including in this term a social community.— It w
I agree with my son's argument & not with rejoinder. The cause of
our difference, I think, is, that I look at the number of offspring as
an important element (all circumstances remaining the same) in
keeping up the average number of individuals within any area.— I do
not believe that the amount of food by any means is the sole
determining cause of number. Lessened fertility is equivalent to a new
source of destruction. I believe if in one district a species produced
from any cause fewer young the deficiency w
I demur to probability & almost to possibility of Par. (1), as you start with two forms, within the same area, which are not mutually sterile, & which yet have supplanted the parent-form.—
(Paragraph 6.) I know of no ghost of a fact supporting belief that disinclination to cross accompanies sterility. It cannot hold with plants, or the lower fixed aquatic animals. I saw clearly what an immense aid this would be, but gave it up. Disinclination to cross seems to have been independently acquired probably by nat. selection; & I do not see why it would not have sufficed to have prevented incipient species from blending to have simply increased sexual disinclination to cross.—
(Par. 11) I demur to a certain extent to amount of sterility & structural dissimilarity necessarily going together, except indirectly & by no means strictly.— Look at vars. of Pigeons, Fowls & Cabbages.—
I overlooked advantage of the half-sterility of reciprocal crosses; yet, perhaps from novelty, I do not feel inclined to admit probability of nat. selection having done its work so queerly.—
I will not discuss second case of utter sterility; but your assumptions in Par. 13 seem to me much too complicated. I cannot believe so universal an attribute as utter sterility between remote species, was acquired in so complex a manner. I do not agree with your rejoinder on grafting: I fully admit that it is not so closely restricted as crossing; but this does not seem to me to weaken the case as one of analogy. The incapacity of grafting is likewise an invariable attribute of plants sufficiently remote from each other, & sometimes of plants pretty closely allied.—
The difficulty of increasing the sterility through nat. selection of two already sterile species seems to me best brought home by considering an actual case. The cowslip & Primrose are moderately sterile, yet occasionally produce hybrids: now, these hybrids, two or three or a dozen in a whole Parish, occupy ground, which might have been occupied by either pure species, & no doubt the latter suffer to this small extent. But can you conceive that any individual plants of the Primrose & cowslip, which happened to be mutually rather more sterile (i.e. which when crossed yielded a few less seed) than usual, would profit to such a degree as to increase in number to the ultimate exclusion of the present primrose & cowslip.— I cannot.—
My son, I am sorry to say, cannot see full force of your rejoinder, in regard to second head of continually augmented sterility. You speak in this rejoinder & in Par. (5) of all the individuals becoming in some slight degree sterile in certain districts; if you were to admit that by continued exposure to these same conditions the sterility would inevitably increase there would no need of Nat. Selection. But I suspect that the sterility is not caused so much by any particular conditions, as by long habituation to conditions of any kind. To speak according to Pangenesis the gemmules of Hybrids are not injured, for Hybrids propagate freely by buds; but their reproductive organs are somehow affected so that they cannot accumulate the proper gemmules, in nearly same manner as the reproductive organs of a pure species become affected when exposed to unnatural conditions.
This is a very ill-expressed & ill-written letter— Do not answer it, unless the spirit urges you— Life is too short for so long a discussion— We shall, I greatly fear, never agree.— My dear Wallace | Most sincerely yours | Ch. Darwin
- f1 6095.f1The year is established by the relationship between this letter and the letter to A. R. Wallace, 27 March .
- f2 6095.f2CD refers to Wallace's argument that hybrid sterility could result from natural selection (see letter from A. R. Wallace, 1 March 1868 and enclosure).
- f3 6095.f3Joseph Dalton Hooker.
- f4 6095.f4George Howard Darwin had replied to some of the points of Wallace's argument on the selection of hybrid sterility and Wallace had responded to the critique (see enclosure to letter to A. R. Wallace, [21 March 1868], and letter from A. R. Wallace, 24 March ).
- f5 6095.f5See enclosure to letter from A. R. Wallace, 1 March 1868.
- f6 6095.f6Herbert Spencer.
- f7 6095.f7See letter from A. R. Wallace, 24 March  and n. 5.
- f8 6095.f8See letter from A. R. Wallace, 24 March  and enclosure. CD was studying the effects of different environments on self-sterility in plants like Eschscholzia californica and had exchanged seeds of this plant with Fritz Müller in Brazil (see letter to Fritz Müller, 3 April  and n. 2). CD refers to his provisional hypothesis of heredity, pangenesis (see Variation 2: 357--404). CD had suggested that minute particles (gemmules) circulated in the bodily fluids and were capable of generating new cells, remaining dormant until required. He thought his hypothesis could explain both sexual and asexual reproduction, as well as reversion and the regrowth of body parts.