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:² 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^d be most grateful for the benefit of them.—

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^d give you such a history of my doings, it w^d be hard to say) from such facts, as the relationship between the living & extinct mammifers in S. America, & between those living on the continent & on adjoining islands, such as the Galapagos— It occurred to me, that a collection of all such analogous facts would throw light either for or against the view of related species, being co-descendants from a common stock. A long searching amongst agricultural & horticultural books & people, makes me believe (I well know how absurdly presumptuous this must appear) that I see the way in which new varieties become exquisitely adapted to the external conditions of life, & to other surrounding beings.— I am a bold man to lay myself open to being thought a complete fool, & a most deliberate one.— From the nature of the grounds, which make me believe that species are mutable in form, these grounds cannot be restricted to the closest-allied species; but how far they extend, I cannot tell, as my reasons fall away by degrees, when applied to species more & more remote from each other.

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)³ of my conclusions; & if I thought at some future time, that you would think it worth reading, I sh^d. of course be most thankful to have the criticism of so competent a critic.

Excuse this very long & egotistical & ill written letter, which by your remarks you have led me into, & believe me, Yours very truly | C. Darwin