As has been indicated above, Darwin suggested a cause of variation in the direct effect of changed external conditions on the reproductive system.
In comparing the two essays it is not unnatural to conclude, as Professor Osborn has done (“From the Greeks to Darwin,” 1894, p. 245), that the two writers held different views upon the material utilised by natural selection in the production of new species, Darwin relying upon the usual slight differences which separate individuals and upon variations in single characters, Wallace upon fully formed varieties—viz. individuals which departed conspicuously from the type of the species, and which may exist singly or in considerable numbers side by side with the parent form.
Professor Osborn’s actual words are as follows:—
“Darwin dwells upon variations in single characters, as taken hold of by Selection; Wallace mentions variations, but dwells upon full-formed varieties, as favourably or unfavourably adapted. It is perfectly clear that with Darwin the struggle is so intense that the chance of survival of each individual turns upon a single and even slight variation. With Wallace, varieties are already presupposed by causes which he does not discuss, a change in the environment occurs, and those varieties which happen to be adapted to it survive. There is really a wide gap between these two statements and applications of the theory.”
Further consideration tends to obliterate this supposed distinction. Although Wallace used the term “variety” as contrasted with “species,” the whole context proves that he, equally with Darwin, recognised the importance of individual variations and of variations in single characters. This becomes clear when we remember his argument about the neck of the giraffe, the changes of colour and hairiness, the shorter legs of the antelope, and the less powerful wings of the passenger pigeon. Wallace has kindly written to me (May 12th, 1896) stating the case as I have given it, and he further explains—
“I used the term ‘varieties’ because ‘varieties’ were alone recognised at that time, individl variability being ignored or thought of no importance. My ‘varieties’ therefore included ‘individual variations.’”
On the other hand, Darwin certainly included large single variations (in other words, “varieties”) as well as ordinary individual differences, among the material for natural selection, and he did not abandon the former until he was convinced by the powerful reasoning of Fleeming Jenkin (North British Review, June, 1867), who argued that single large differences of a sudden and conspicuous kind (Darwin’s “variations”) would certainly be swamped by intercrossing. Upon this review of the “Origin” Francis Darwin says (“Life and Letters”)—
“It is not a little remarkable that the criticisms which my father, as I believe, felt to be the most valuable ever made on his views should have come, not from a professed naturalist but from a Professor of Engineering.”
After reading this review, Darwin wrote to Wallace (January 22nd, 1869):—
“I always thought individual differences more important than single variations, but now I have come to the conclusion that they are of paramount importance, and in this I believe I agree with you. Fleeming Jenkin’s arguments have convinced me.”