February 14th [1845].
I have taken my leisure in thanking you for your last letter and discussion, to me very interesting, on the increase of species.

Since your letter, I have met with a very similar view in Richardson, who states that the young are driven away by the old into unfavourable districts, and there mostly perish.

When one meets with such unexpected statistical returns on the increase and decrease and proportion of deaths and births amongst mankind, and in this well-known country of ours, one ought not to be in the least surprised at one's ignorance, when, where, and how the endless increase of our robins and sparrows is checked.
Thanks for your hints about terms of "mutation," etc.; I had some suspicions that it was not quite correct, and yet I do not see my way to arrive at any better terms.

It will be years before I publish, so that I shall have plenty of time to think of better words.

Development would perhaps do, only it is applied to the changes of an individual during its growth.