1 845.] STRUGGLE FOR LIFE. 33 



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 par- 

 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 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 con- 

 jecture 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 suc- 

 ceeding 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 



* Rev. L. Blomefield. 

 VOL. II. D 



