To Leonard Jenyns 12 October 1
Down Bromley Kent
My dear Jenyns
Thanks for your note.— I am sorry to say that 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 & energy, both of which ingredients I have had none spare, as writing my geology thoroughily expends both. I had always thought, that I would keep a journal & record everything, but in the way I now live I find I observe nothing to record. Looking after my garden & trees & occasionally a very little walk, in an idle frame of my mind, fills up every afternoon in the same manner.—
I am surprised that with all your parish affairs that you have had time to do all, that which you have done. I shall be very glad to see your little work2 (& proud shd. 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.3 These are the facts, which make one understand the working or œconomy of nature. There is one subject, on which I am very curious, & which perhaps you may throw some light on, if you have ever thought on it,—namely what are the checks & 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 & these breed: within the natural ie. if free from accidents life of the parents, the number of individuals will become enormous, & I have been much surprised to think, how great destruction must annually or occasionally be falling on every species, yet the means & period of such destruction scarcely perceived by us.
I have continued steadily reading & collecting facts on variation of domestic animals & plants & on the question of what are species; I have a grand body of facts & I think I can draw some sound conclusions. The general conclusion at which I have slowly been driven from a directly opposite conviction is that species are mutable & that allied species are co-descendants of common stocks. I know how much I open myself, to reproach, for such a conclusion, but I have at least honestly & deliberately come to it.
I shall not publish on this subject for several years— At present I am on the geology of S. 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
Asks whether LJ can throw light on this subject: "What are the checks and what the periods of life by which the increase of any given species is limited?" CD has been driven to conclude that species are mutable; allied species are co-descendants from common stocks.