My son, I am sorry to say, cannot see the full force of your rejoinder in regard to the second head of continually augmented sterility.  You speak in this rejoinder, and in par. 5, of all the individuals becoming in some slight degree sterile in certain districts; if you were to admit that by continued exposure to these same conditions the sterility would inevitably increase, there would be no need of Natural Selection.  But I suspect that the sterility is not caused so much by any particular conditions, as by long habituation to conditions of any kind.  To speak according to pangenesis, the gemmules of hybrids are not injured, for hybrids propagate freely by buds; but their reproductive organs are somehow affected, so that they cannot accumulate the proper gemmules, in nearly 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?