apted to any special sphere of
existence as the pure species a and b, they would certainly in their
turn give way to a and b.

LETTER 210. TO A.R. WALLACE. February 27th [1868].

I shall be very glad to hear, at some future day, your criticisms on
the "causes of variability." Indeed, I feel sure that I am right about
sterility and Natural Selection. 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. Now try and make (by Natural Selection) A and B absolutely
sterile when crossed, and you will find how difficult it is. I
grant, indeed it is certain, that the degree of the sterility of the
individuals of A and B will vary; but any such extra-sterile individuals
of, we will say A, if they should hereafter breed with other individuals
of A, will bequeath no advantage to their progeny, by which these
families will tend to increase in number over other families of A, which
are not more sterile when crossed with B. But I do not know that I
have made this any clearer than in the chapter in my book. It is a most
difficult bit of reasoning, which I have gone over and over again on
paper with diagrams. (210/1. This letter appeared in "Life and Letters,"
III., page 80.)

LETTER 211. 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, th