r consists in taking the rate of deposition as the same as the
rate of denudation, whereas it is about twenty times as great, perhaps
much more--because the area of deposition is at least twenty times less
than that of denudation. In order to equal the area of denudation, it
would require that _every_ bed of _every_ formation should have once
extended over the _whole area_ of all the land of the globe! The
deposition in narrow belts along coasts of all the matter brought down
by rivers, as proved by the _Challenger_, leads to the same result. In
my "Island Life," 2nd Edit., pp. 221-225, I have discussed this whole
matter, and on reading it again I can find no fallacy in it. I have,
however, I believe, overestimated the time required for deposition,
which I believe would be more nearly one-fortieth than one-twentieth
that of mean denudation; because there is, I believe, also a great
overestimate of the maximum of deposition, because it is partly made up
of beds which may have been deposited simultaneously. Also the maximum
thickness is probably double the mean thickness.

The mean rate of denudation, both for European rivers and for all the
rivers that have been measured, is a foot in three million years, which
is the figure that should be taken in calculations.--Believe me yours
very truly,

ALFRED R. WALLACE.

* * * * *

TO PROF. MELDOLA

_Parkstone, Dorset. April 27, 1897._

My dear Meldola,-- ... I thought Romanes' article in reply to Spencer
was very well written and wonderfully clear for him, and I agree with
most of it, except his high estimate of Spencer's co-adaptation
argument. It is quite true that Spencer's biology rests entirely on
Lamarckism, so far as heredity of acquired characters goes. I have been
reading Weismann's last book, "The Germ Plasm." It is a wonderful
attempt to solve the most complex of all problems, and is almost
unreadable without some practical acquaintance with germs and their
development.--Believe me yours very faithfully,

ALFRED R. WALLACE.

* * * * *

TO PROF. POULTON

_Parkstone, Dorset. June 13, 1897._

My dear Poulton,-- ... The rate of deposition might be modified in an
archipelago, but would not necessarily be less than now, on the
_average_. On the ocean side it might be slow, but wherever there were
comparatively narrow straits between the islands it might be even faster
than now, because the ar