On checks to increase of species and the observations which led him to regard species as mutable in form. Would welcome "at some future time" LJ's criticism of the "sketch" of his conclusions.
Down Bromley Kent
My dear Jenyns,
I am very much obliged to you for the trouble you have taken in having written me so long a note. The question of where, when, & how, the check to the increase of a given species falls appears to me particularly interesting; & our difficulty in answering it, shows how really ignorant we are of the lives & habits of our most familiar species. I was aware of the bare fact of old Birds driving away their young, but had never thought of the effect, you so clearly point out, of local gaps in number being thus immediately filled up. But the original difficulty remains, for if your farmers had not killed your sparrows & rooks, what would have become of those, which now immigrate into your Parish: in the middle of England one is too far distant from the natural limits of the Rook & sparrow, to suppose that the young are thus far expelled from Cambridgeshire. The check must fall heavily at some time of each species's life, for if one calculates that only half the progeny are reared & breed,—how enormous is the increase! One has, however, no business to feel so much surprise at one's ignorance, when one knows how impossible, it is, without statistics, to conjecture the duration of life & percentage of deaths to births in mankind.
If it could be shown that apparently the birds of passage, which breed here
& increase return in the succeeding years in about the same number, whereas
those that come here for their winter—& non-breeding season, annually
come here with the same numbers, but return with greatly decreased numbers, one would
know (as indeed seems probable) that the check fell chiefly on full-grown birds in the
winter season, & not on the eggs & very young birds, which has
appeared to me often the most probable period. If at any time any remarks on this
subject should occur to you, I sh
With respect to my far-distant work on species, I must have expressed myself with
singular inaccuracy, if I led you to suppose that I meant to say that my conclusions
were inevitable. They have become so, after years of weighing puzzles, to myself
alone;; but in my wildest day-dream, I never expect more than to be able to
show that there are two sides to the question of the immutability of species, ie whether
species are directly created, or by intermediate laws, (as with the life
& death of individuals). I did not approach the subject on the side of the
difficulty in determining what are species & what are varieties, but (though,
why I sh
Pray do not think, that I am so blind as not to see that there are numerous immense
difficulties on my notions, but they appear to me less than on the common
view.— I have drawn up a sketch & had it copied (in
200 pages) of my conclusions; & if I thought
at some future time, that you would think it worth reading, I
Excuse this very long & egotistical & ill written letter, which by your remarks you have led me into, & believe me, Yours very truly | C. Darwin
- f1 793.f1This letter follows the letter to Leonard Jenyns, 12 October . November 25 was the only following ‘Monday 25
- f2 793.f2In Natural selection, p. 185, CD referred to Jenyns on this point and made the same comment.
- f3 793.f3The ‘fair copy’ of the essay of 1844 is in DAR 113; the original manuscript is in DAR 7. There is no record that Jenyns ever read the essay.