the same manner as the reproductive organs of a pure species
become affected when exposed to unnatural conditions.

This is a very ill-expressed and ill-written letter. Do not answer it,
unless the spirit urges you. Life is too short for so long a discussion.
We shall, I _greatly_ fear, never agree.--My dear Wallace, most
sincerely yours,

CH. DARWIN.

* * * * *

_Hurstpierpoint. [?] April 8, 1868._

Dear Darwin,--I am sorry you should have given yourself the trouble to
answer my ideas on Sterility. If you are not convinced, I have little
doubt but that I am wrong; and in fact I was only _half convinced_ by my
own arguments, and I now think there is about an even chance that
Natural Selection may or not be able to accumulate sterility. If my
first proposition is modified to _the existence of a species and a
variety in the same area_, it will do just as well for my argument. Such
certainly do exist. They are fertile together, and yet each maintains
itself tolerably distinct. How can this be, if there is no
disinclination to crossing? My belief certainly is that number of
offspring is not so important an element in keeping up population of a
species as supply of food and other favourable conditions, because the
numbers of a species constantly vary greatly in different parts of its
area, whereas the average number of offspring is not a very variable
element.

However, I will say no more but leave the problem as insoluble, only
fearing that it will become a formidable weapon in the hands of the
enemies of Natural Selection.

While writing a few pages on the northern alpine forms of plants on the
Java mountains I wanted a few cases to refer to like Teneriffe, where
there are no _northern_ forms, and scarcely any alpine. I expected the
volcanoes of Hawaii would be a good case, and asked Dr. Seeman about
them. It seems a man has lately published a list of Hawaiian plants, and
the mountains swarm with European alpine genera and some species![68] Is
not this most extraordinary and a puzzler? They are, I believe, truly
oceanic islands in the absence of mammals and the extreme poverty of
birds and insects, and they are within the tropics. Will not that be a
hard nut for you when you come to treat in detail on geographical
distribution?

I enclose Seeman's note, which please return when you have copied the
list, if of any use to you.

Many thanks for your carte, which I think very good. T