ations of organic
beings, this coincidence must have taken place an almost infinite number
of times.

Now it seems to me that you have yourself led to this objection being
made by so often stating the case too strongly against yourself. For
example, at the commencement of Chapter IV. you ask if it is "improbable
that useful variations should sometimes occur in the course of thousands
of generations"; and a little further on you say, "unless profitable
variations do occur, natural selection can do nothing." Now, such
expressions have given your opponents the advantage of assuming that
_favourable_ variations are _rare accidents_, or may even for long
periods never occur at all, and thus Janet's argument would appear to
many to have great force. I think it would be better to do away with all
such qualifying expressions, and constantly maintain (what I certainly
believe to be the fact) that _variations of every kind_ are _always
occurring_ in _every part_ of _every species_, and therefore that
favourable variations are _always ready_ when wanted. You have, I am
sure, abundant materials to prove this, and it is, I believe, the grand
fact that renders modification and adaptation to conditions almost
always possible. I would put the burthen of proof on my opponents to
show that any one organ, structure, or faculty does _not vary_, even
during one generation, among all the individuals of a species; and also
to show any _mode or way_ in which any such organ, etc., does not vary.
I would ask them to give any reason for supposing that any organ, etc.,
is ever _absolutely identical_ at any _one time in all the individuals_
of a species, and if not, then it is always varying, and there are
always materials which, from the simple fact that the "fittest survive,"
will tend to the modification of the race into harmony with changed
conditions.

I hope these remarks may be intelligible to you, and that you will be so
kind as to let me know what you think of them.

I have not heard for some time how you are getting on. I hope you are
still improving in health, and that you will be able now to get on with
your great work, for which so many thousands are looking with
interest.--With best wishes, believe me, my dear Darwin, yours very
faithfully,

ALFRED R. WALLACE.

* * * * *

_Down, Bromley, Kent, S.E. July 5, [1866]._

My dear Wallace,--I have been much interested by your letter, which is
as clear