In order that any great amount of modifications should be effected by a species, a variety when once formed must again perhaps after a long interval of time, vary or present individual differences of the same favorable nature as before; and then must again be preserved and so onward step by step.
Those lines contain an abstract of de Vries's mutation theory. And then further, on page 72:
It should not, however, be overlooked that certain rather strongly marked variations, which no one would rank as mere individual differences, frequently occur owing to a similar organization being similarly acted on. . . . There can also be little doubt that the tendency to vary in the same manner has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection. Or only a third, fifth or tenth part of the individuals may have been thus affected, of which fact several instances could be given. For cases of this kind, if the variations were of a beneficial nature, the original form would soon be supplanted by the modified form, through the survival of the fittest.
Again, when Darwin denies having said that time alone plays a part
in the process of modification which changes one species into another, he writes (p. 82, l. c.):
Lapse of time is only so far important, and its importance in this respect is great, that it gives a better chance of beneficial variations arising and of their being selected, accumulated and fixed.
In the fragments which I have quoted Darwin appears to have had before his mind mutation, not fluctuating variation. And I must insist on the fact that de Vries makes a point of showing that Darwin was decidedly inclined to accept the process of mutation. De Vries quotes (p. 25) from Darwin's 'Life and Letters' (p. 87, Vol. II.) and from the 'Origin of Species,' e. g., the following words:
The formation of a. . . species I look at as almost wholly due to the selection of what may be incorrectly called chance variations. . . unless such occur, natural selection can do nothing, and he adds: It is obvious that Darwin has attributed a great and often a preponderating, perhaps even an exclusive significance to the single variations. . . .
The chance variations were not for Darwin the extreme cases of fluctuating variability, that can be everywhere observed; they were fortuitous phenomena. For these natural selection is always on the lookout, or as Darwin has it, metaphorically, 'He catches hold of them, whenever and wherever opportunity offers.' Darwin must have been inclined to think that these variations, these mutations, arise in accordance with certain laws which are entirely unknown to us. In consequence of the operation of these laws at least a certain number of favorable modifications must inevitably arise after a given lapse of time. Hence the gradual evolution which most living organisms have undergone in the course of ages.
Darwin also undoubtedly suspected the existence of a certain periodicity. 'Nascent species are more plastic,' he says; and he
