ther well-marked races have often been produced by the
direct action of changed conditions without the aid of selection either
by man or nature." ("Animals and Plants," Volume II., page 292, 1868.)),
I intended to refer to the direct action of such conditions in causing
variation, and not as leading to the preservation or destruction of
certain forms. There is as wide a difference in these two respects
as between voluntary selection by man and the causes which induce
variability. I have somewhere in my book referred to the close
connection between Natural Selection and the action of external
conditions in the sense which you specify in your note. And in this
sense all Natural Selection may be said to depend on changed conditions.
In the "Origin" I think I have underrated (and from the cause which
you mention) the effects of the direct action of external conditions in
producing varieties; but I hope in Chapter XXIII. I have struck as fair
a balance as our knowledge permits.

It is wonderful to me that you have patience to read my slips, and I
cannot but regret, as they are so imperfect; they must, I think, give
you a wrong impression, and had I sternly refused, you would perhaps
have thought better of my book. Every single slip is greatly altered,
and I hope improved.

With respect to the human ovule, I cannot find dimensions given, though
I have often seen the statement. My impression is that it would be just
or barely visible if placed on a clear piece of glass. Huxley could
answer your question at once.

I have not been well of late, and have made slow progress, but I think
my book will be finished by the middle of November.

LETTER 218. A.R. WALLACE TO CHARLES DARWIN. [End of February, 1868]

I am in the second volume of your book, and I have been astonished at
the immense number of interesting facts you have brought together. I
read the chapter on pangenesis first, for I could not wait. I can hardly
tell 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 S