ell you how much I admire it. It is a positive _comfort_ to me to have
any feasible explanation of a difficulty that has always been haunting
me, and I shall never be able to give it up till a better one supplies
its place, and that I think hardly possible. You have now fairly beaten
Spencer on his own ground, for he really offered no solution of the
difficulties of the problem. The incomprehensible minuteness and vast
numbers of the physiological germs or atoms (Which themselves must be
compounded of numbers of Spencer's physiological units) is the only
difficulty, but that is only on a par with the difficulties in all
conceptions of matter, space, motion, force, etc. As I understood
Spencer, his physiological units were identical throughout each species,
but slightly different in each different species; but no attempt was
made to show how the identical form of the parent or ancestors came to
be built up of such units.

The only parts I have yet met with where I somewhat differ from your
views are in the chapter on the Causes of Variability, in which I think
several of your arguments are unsound: but this is too long a subject to
go into now.

Also, I do not see your objection to _sterility_ between allied species
having been aided by Natural Selection. It appears to me that, given a
differentiation of a species into two forms, each of which was adapted
to a special sphere of existence, every slight degree of sterility would
be a positive advantage, not to the _individuals_ who were sterile, but
to _each form_. If you work it out, and suppose the two incipient
species A, B to be divided into two groups, one of which contains those
which are fertile when the two are crossed, the other being slightly
sterile, you will find that the latter will certainly supplant the
former in the struggle for existence, remembering that you have shown
that in such a cross the offspring would be _more vigorous_ than the
pure breed, and would therefore certainly soon supplant them, and as
these would not be so well adapted 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.

I am sure all naturalists will be disgusted at the malicious and
ignorant article in the _Athenaeum_. It is a disgrace to the paper, and I
hope someone will publicly express the general opinion of it. We can
expect no good reviews of your book till the quarterlies or best
monthlies come out.... I sh