dified to prevent crossing.

It is this which makes me so much interested with dimorphism, etc.
(152/2. This gives a narrow impression of Darwin's interest in
dimorphism. The importance of his work was (briefly put) the proof that
sterility has no necessary connection with specific difference, but
depends on sexual differentiation independent of racial differences. See
"Life and Letters," III., page 296. His point of view that sterility
is a selected quality is again given in a letter to Huxley ("Life and
Letters," II., page 384), but was not upheld in his later writings (see
"Origin of Species," Edition VI., page 245). The idea of sterility being
a selected quality is interesting in connection with Romanes' theory of
physiological selection. (See Letters 209-214.))

One word more. When you pitched me head over heels by your new way of
looking at the back side of variation, I received assurance and strength
by considering monsters--due to law: horribly strange as they are, the
monsters were alive till at least when born. They differ at least as
much from the parent as any one mammal from another.

I have just finished a long, weary chapter on simple facts of variation
of cultivated plants, and am now refreshing myself with a paper on Linum
for the Linnean Society.

LETTER 153. TO W.B. TEGETMEIER.

(153/1. The following letter also bears on the question of the
artificial production of sterility.)

Down, 27th [December, 1862].

The present plan is to try whether any existing breeds happen to
have acquired accidentally any degree of sterility; but to this point
hereafter. The enclosed MS. will show what I have done and know on the
subject. Please at some future time carefully return the MS. to me. If I
were going to try again, I would prefer Turbit with Carrier or Dragon.

I will suggest an analogous experiment, which I have had for two years
in my experimental book with "be sure and try," but which, as my health
gets yearly weaker and weaker and my other work increases, I suppose
I shall never try. Permit me to add that if 5 pounds would cover the
expenses of the experiment, I should be delighted to give it, and you
could publish the result if there be any result. I crossed the Spanish
cock (your bird) and white Silk hen and got plenty of eggs and chickens;
but two of them seemed to be quite sterile. I was then sadly overdone
with work, but have ever since much reproached myself that I did not
preserve and care