i845.] STRUGGLE FOR LIFE. 393 



on every species, yet the means and period of such destruc- 

 tion 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 con- 

 clusions 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 con- 

 clusion, 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 frorn 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 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 v/hich 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. 



