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1845.)

MUTABILITY OF SPECIES.

391

letter till to-morrow, for I am tired ; but I so enjoy writing to you, that I must inflict a little more on you.

Have you any good evidence for absence of insects in small islands? I found thirteen species in Keeling Atoll. Flies are good fertilizers, and I have seen a microscopic Thrips and a Cecidomya take flight from a flower in the direction of another with pollen adhering to them. In Arctic countries a bee seems to go as far N. as any flower.

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C. Darwin to J. D. Hooker.

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Slirewsbury (September, 1845]. MY DEAR HOOKER,- I write a line to say that Cosmos arrived quite safely (N.B. One sheet came loose in Pt. I.), and to thank

you
for
your
nice note.

I have just begun the introduction, and groan over the style, which in such parts is full half the battle. How true many of the remarks are (i.e. as far as I can understand the wretched English) on the scenery; it is an exact expression of one's own thoughts.

I wish I ever had any books to lend you in return for the many you have lent me.

All of what you kindly say about my species work does not alter one iota my long self-acknowledged presumption in accumulating facts and speculating on the subject of variation, without having worked out my due share of species. But now for nine years it has been anyhow the greatest aniusement to me.

Farewell, my dear Hooker, I grieve more than you can well believe, over our prospect of so seldom meeting.

I have never perceived but one fault in you, and that you have grievously, viz. modesty ; you form an exception to Sydney Smith's aphorism, that merit and modesty have no other connection, except in their first letter. Farewell,

C. DARWIN.

* A translation of Humboldt's 'Kosmos.'

C. Darwin to L. Jenyns (Blome field).

Down, Oct. 12th, (1845). MY DEAR JENYNS, –Thanks for your note. I am sorry to say I have not even the tail-end of a fact in English Zoology to communicate. I have found that even trifling observations require, in my case, some leisure and energy, both of which ingredients I have had none to spare, as writing my Geology thoroughly expends both. I had always thought that I would keep a journal and record everything, but in the way I now live I find I observe nothing to record. Looking after my garden and trees, and occasionally a very little walk in an idle frame of mind, fills up every afternoon in the same man

I am surprised that with all your parish affairs, you have had time to do all that which you have done. I shall be very glad to see your little work * (and proud should I have been if I could have added a single fact to it). My work on the species question has impressed me very forcibly with the importance of all such works as your intended one, containing what people are pleased generally to call trifling facts. These are the facts which make one understand the working or economy of nature. There is one subject, on which I am very curious, and which perhaps you may throw some light on, if you have ever thought on it; namely, what are the checks and what the periods of life, -by which the increase of any given species is limited. Just calculate the increase of any bird, if you assume that only half the young are reared, and these breed : within the natural (i. e., if free from accidents) life of the parents the number of individuals will become enormous, and I have been much surprised to think how great destruction must annually or occasionally be falling

* Mr. Jenyns' 'Observations in Natural History.' It is prefaced by an Introduction on "Hadits of observing as connected with the study of Natural History," and followed by a “Calendar of Periodic Phenomena in Natural History," with “Remarks on the importance of such Registers." My father seems to be alluding to this Register in the P.Ş. to the letter dated Oct. 17, 1846.

1845.]

STRUGGLE FOR LIFE.

393

on every species, yet the means and period of such destruction is scarcely perceived by us.

I have continued steadily reading and collecting facts on variation of domestic animals and plants, and on the question of what are species. I have a grand body of facts, and I think I can draw some sound conclusions, The general conclusions at which I have slowly been driven from a directly opposite conviction, is that species are mutable, and that allied species are co-descendants from common stocks. I know how much I open myself to reproach for such a conclusion, but I have at least honestly and deliberately come to it. I shall not publish on this subject for several years. At present I am on the Geology of South America. I hope to pick up from your book some facts on slight variations in structure or instincts in the animals of your acquaintance.

Believe me, ever yours,

C. DARWIN.

C. Darwin to L. Jenyns.

Down, (1845?). 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, and how the check to the increase of a given species falls appears to me para ticularly interesting, and our difficulty in answering it shows how really ignorant we are of the lives and 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 and 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 and sparrow to suppose

* Rev. L. Blomefield.

that the young are thus far expelled from Cambridgeshire. The check must fall heavily at some time of each species' life; for, if one calculates that only half the progeny are reared and bred, 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 and percentage of deaths to births in mankind. If it could be shown that apparently the birds of passage which breed here and increase, return in the succeeding years in about the same number, whereas those that come here for their winter and 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, and not on the eggs and 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 should 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, i. e. whether species are directly created or by intermediate laws (as with the life and death of individuals). I did not approach the subject on the side of the difficulty in determining what are species and what are varieties, but (though, why I should give you such a history of my doings it would be hard to say) from such facts as the relationship between the living and extinct mammifers in South America, and between those living on the Continent and 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

1846.]

MR. JENYNS' OBSERVATIONS.'

395

stock. A long searching amongst agricultural and horricultural books and 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 and to other surrounding beings. I am a bold man to lay myself open to being thought a complete fool, and 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 closestallied species ; but how far they extend I cannot tell, as my reasons fall away by degrees, when applied to species more and more remote from each other. Pray do not think that I am so blind as not to see that there are numerous immense difficulties in my notions, but they appear to me less than on the common view. I have drawn up a sketch and had it copied (in 200 pages) of my conclusions; and if I thought at some future time that you would think it worth reading, I should, of course, be most thankful to have the criticism of so competent a critic. Excuse this very long and egotistical and ill-written letter, which by your remarks you had led me into, and believe me,

Yours very truly,

C. DARWIN.

C. Darwin to I. Jenyns.

Down, Oct. 17th, 1846. DEAR JENYNS, I have taken a most ungrateful length of time in thanking you for your very kind present of your 'Observations.' But I happened to have had in hand several other books, and have finished yours only a few days ago. I found it very pleasant reading, and many of your facts interested me much. I think I was more interested, which is odd, with your notes on some of the lower animals than on the higher ones. The introduction struck me as very good ; but this is what I expected, for I well remember being quite delighted with a preliminary essay to the first number of the 'Annals of Natural History.' I missed one discussion, and

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