more carefully,
as I hope hereafter to do, all your papers; but I shall feel much more
confidence in a brief reply from you. Am I right in supposing that you
believe that the glacial periods have always occurred alternately in the
northern and southern hemispheres, so that the erratic deposits which I
have described in the southern parts of America, and the glacial work in
New Zealand, could not have been simultaneous with our Glacial period?
From the glacial deposits occurring all round the northern hemisphere,
and from such deposits appearing in S. America to be as recent as in the
north, and lastly, from there being some evidence of the former lower
descent of glaciers all along the Cordilleras, I inferred that the whole
world was at this period cooler. It did not appear to me justifiable
without distinct evidence to suppose that the N. and S. glacial deposits
belonged to distinct epochs, though it would have been an immense
relief to my mind if I could have assumed that this had been the
case. Secondly, do you believe that during the Glacial period in one
hemisphere the opposite hemisphere actually becomes warmer, or does
it merely retain the same temperature as before? I do not ask these
questions out of mere curiosity; but I have to prepare a new edition
of my "Origin of Species," and am anxious to say a few words on this
subject on your authority. I hope that you will excuse my troubling you.

LETTER 510. TO J. CROLL. Down, January 31st, 1869.

To-morrow I will return registered your book, which I have kept so long.
I am most sincerely obliged for its loan, and especially for the MS.,
without which I should have been afraid of making mistakes. If you
require it, the MS. shall be returned. Your results have been of more
use to me than, I think, any other set of papers which I can remember.
Sir C. Lyell, who is staying here, is very unwilling to admit the
greater warmth of the S. hemisphere during the Glacial period in the N.;
but, as I have told him, this conclusion which you have arrived at from
physical considerations, explains so well whole classes of facts in
distribution, that I must joyfully accept it; indeed, I go so far as to
think that your conclusion is strengthened by the facts in distribution.
Your discussion on the flowing of the great ice-cap southward is
most interesting. I suppose that you have read Mr. Moseley's recent
discussion on the force of gravity being quite insufficient to account
for