From A. R. Wallace 8 [April] 18681
March 8th. | 1868
I am sorry you should have given yourself the trouble to answer my ideas on Sterility—2 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 Nat. Select. may or not be able to accumulate sterility. If my 1st prop. is modified to,—the existence of a species and a variety in the same area, it will do just as well for my argument.3 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 Nat. 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. Seemann4 about them. It seems a man has lately published a list of Hawaaian plants and the mountains swarm with European alpine genera & some species!.5 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 & Insects and they are within the tropics.
Will not that be a hard nut for you when you come to treat in detail on Geog. distribution.
I enclose Seemann’s note, which please return when you have copied the list if of any use to you.6
Many thanks for your carte which I think very good. The large one had not arrived when I was in town last week.7
Sir C. Lyell’s Chap. on Oceanic Is. I think very good.8
Believe me Dear Darwin | Yours very faithfully | Alfred R. Wallace—
If CD is not convinced by his notes on sterility, ARW has little doubt that he is wrong. In fact he was only half-convinced by his own arguments.
Modifies his first proposition [a species varies occasionally in two directions, but owing to free inter-crossing the variations never increase] and further discusses the subject.
Encloses Berthold Seemann’s notes on flora of the Hawaiian Islands. Presence of European alpine species in Hawaiian volcanoes is a "hard nut" for geographical distribution [but see ARW’s Island life (1880), p. 323].