pencer'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.

LETTER 219. TO A.R. WALLACE. Down, February 27th [1868].

You cannot well imagine how much I have been pleased by what you say
about pangenesis. None of my friends will speak out, except to a certain
extent Sir H. Holland, who found it very tough reading, but admits that
some view "closely akin to it" will have to be admitted. Hooker, as far
as I understand him, which I hardly do at present, seems to think that
the hypothesis is little more than saying that organisms have such
and such potentialities. What you say exactly and fully expresses my
feelings--viz., that it is a relief to have some feasible explanation
of the various facts, which can be given up as soon as any better
hypothesis is found. It has certainly been an immense relief to my mind;
for I have been stumbling over the subject for years, dimly seeing that
some relation existed between the various classes of facts. I now hear
from H. Spencer that his views quoted in my footnote refer to something
quite distinct, as you seem to have perceived. (219/1. This letter is
published in "Life and Letters," III., page 79.)

LETTER 220. A.R. WALLACE TO CHARLES DARWIN. Hurstpierpoint, March 1st,
1868.

...Sir C. Lyell spoke to me as if he has greatly admired pangenesis. I
am very glad H. Spencer at once acknowledges that his view was something
quite distinct from yours. Although, as you know, I am a great admirer
of his, I feel how completely his view failed to go to the root of the
matter, as yours does. His explained nothing, though he was evidently
struggling hard to find an explanation. Yours, as far as I can see,
explains everything in growth and reproduction--though, of course, the
mystery of life and consciousness remains as great as ever.

Parts of the chapter on pangenesis I found hard reading, and have not
quite mastered yet, and there are also throughout the discussions in
Volume II. many bits of hard reading, on minute points which we, who
have not worked experimentally at cultivation and crossing, as you have
done, can hardly see the im