From A. R. Wallace 24 March 1
Many thanks for the Photo. which I shall get when I go to town.2
I return your son’s notes with my notes on them.3
Without going into any details is not this a strong general argument:
1. A species varies occasionally in two directions, but owing to their free intercrossing they the varieties never increase.
2. A change of conditions occurs which threatens the existence of the species,—but the two varieties are adapted to the changing conditions and if accumulated will form two new species adapted to the new conditions.
3. Free crossing however renders this impossible, and so the species is in danger of extinction
4. If sterility could be induced, then the pure races would increase more rapidly and replace the old species.
5. It is admitted that partial sterility between varieties does occasionally occur. It is admitted the degree of this sterility varies; is it not probable that Nat. Select. can accumulate these variations and thus save the species? If Nat. Select. can not do this how do species ever arise, except when a variety is isolated?
Closely allied sp. in distinct countries being sterile is no difficulty;—for either they diverged from a common ancestor in contact, and Nat. Select. increased the sterility;—or they were isolated, and have varied since, in which case they have been for ages influenced by distinct conditions wh. may well produce sterility.4
If the difficulty of grafting was as great as the difficulty of crossing, and as regular, I admit it would be a most serious objection. But it is not. I believe, many distinct species can be grafted while others less distinct cannot.5 The regularity with which Nat. species are sterile together, even when very much alike, I think is an argument in favour of the sterility having been generally produced by Nat. Select. for the good of the species.
The other difficulty, of unequal sterility of reciprocal crosses,—seems none to me; for it is a step to more complete sterility, and as such would be useful and would be increased by selection.6
I have read Sir C. Lyell’s 2nd Vol. with great pleasure.7 He is as usual very cautious and hardly ever expresses a positive opinion, but the general effect of the whole book is very strong, as the argument is all on our side.
I am in hopes it will bring in a new set of converts to Nat. Selection, and will at all events lead to a fresh ventilation of the subject.
Believe me Yours very faithfully | Alfred R. Wallace—
I do not think Mr.____ Darwin’s objection to ppar. 5. is sound.8 He assumes that the total number of individuals will after a time be less in the area where some degree of sterility of hybrids prevails— This would not be the case, for in both areas the number dying annually must equal on an average those that are born. If there are, say a million individuals in each area, there may be two million produced annually in the fertile area—1,900,000 in the Sterile area, and these numbers of old or young must on the average die off annually— But in the fertile area the hybrids will be a larger proportion of the whole than in the sterile area, and as they are by hypothesis less fitted to conditions of life than the pure breeds, it seems to me that the new hybrid population of the fertile area would rather tend to give way to the purer populations of the sterile area, than the contrary: Varieties whose coming into existence as fixed forms has solely depended on their being each adapted to a special place in nature, must I conceive in the long run, gain an advantage over hybrids between them, which, being less specialised are less fitted for the existing conditions.
The objection to this par. seems more serious but I think can be overcome.
Let the offspring in 1st. Generation be
AS + BS + (ABS = 0) and
AF + BF + ABF
In the 2nd. Generation there will be again AS. + BS, in about equal numbers, but AF + BF will be diminished on account of crosses between AFABF and BFABF, and this will go on till there are no pure AF or BF left, but a series of hybrids of various degrees. AS.BS. on the contrary will continue pure, & must I conceive ultimately supplant the hybrids for the same reason as in the last case. It is necessary however I think to suppose the two sets to inhabit distinct areas (such as mountain & valley) otherwise crosses between AS AF and BS BF would eliminate all distinction of fertility and sterility. But as the whole tendency of Mr Darwin’s works is to show that some changes of conditions induce sterility, I think this is a legitimate supposition. Unless this is granted I am afraid the case is hopeless
It must always be remembered that the total population year by year remains stationary. The question is, what portion of the increase has the best charge of surviving.9
Returns George Darwin’s criticisms of his notes on sterility and sends further notes in reply. Since there are degrees of sterility between varieties, "is it not probable that natural selection can accumulate these variations?" Varieties that are adapted to new conditions could then survive and form new species without being isolated.
Please cite as
Darwin Correspondence Project, “Letter no. 6045,” accessed on 13 February 2016, http://www.darwinproject.ac.uk/DCP-LETT-6045