32 GROWTH OF THE 'ORIGIN.' [1845. 



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 miist annually or 

 occasionally be falling 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 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 from your book some facts on slight variations in 

 structure or instincts in the animals of your acquaintance. 



Believe me, ever yours, 



C. Darwin. 



