Two of my grown-up children who are acute reasoners have two or three times at intervals tried to prove me wrong; and when your letter came they had another try, but ended by coming back to my side.

I do not quite understand your case, and we think that a word or two is misplaced.

I wish some time you would consider the case under the following point of view.

If sterility is caused or accumulated through Natural Selection, then, as every degree exists up to absolute barrenness, Natural Selection must have the power of increasing it.

Now take two species A and B, and assume that they are (by any means) half-sterile, i.e., produce half the full number of offspring.