A.R.WALLACE TO CHARLES DARWIN.

March 1st, 1868.
I beg to enclose what appears to me a demonstration on your own principles, that Natural Selection could produce sterility of hybrids.
If it does not convince you, I shall be glad if you will point out where the fallacy lies.

I have taken the two cases of a slight sterility overcoming perfect fertility, and of a perfect sterility overcoming a partial fertility,--the beginning and end of the process.

You admit that variations in fertility and sterility occur, and I think you will also admit that if I demonstrate that a considerable amount of sterility would be advantageous to a variety, that is sufficient proof that the slightest variation in that direction would be useful also, and would go on accumulating.
1.

Let there be a species which has varied into two forms, each adapted to existing conditions (211/1.