394 GROWTH OF THE 'ORIGIN." [1845. 



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 v/ith 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, /. 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 liv- 

 ing 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 



