Again:-

“On the theory of natural selection we can clearly understand the full meaning of that old canon in natural history, ’Natura non facit saltum.’  This canon, if we look only to the present inhabitants of the world is not strictly correct, but if we include all those of past times, it must by my theory be strictly true” (p. 206).

Here the natural interpretation of “by my theory” is “by the theory of descent with modification;” the words “on the theory of natural selection,” with which the sentence opens, lead us to suppose that Mr. Darwin regarded natural selection and descent as convertible terms.  “My theory” was altered to “this theory” in 1872.  Six lines lower down we read, “On my theory unity of type is explained by unity of descent.”  The “my” here has been allowed to stand.

Again:-

“Again, as in the case of corporeal structure, and conformably with my theory, the instinct of each species is good for itself, but has never,” &c. (p. 210).

Who was to see that “my theory” did not include descent with modification?  The “my” here has been allowed to stand.

Again:-

“The fact that instincts . . . are liable to make mistakes;—­that no instinct has been produced for the exclusive good of other animals, but that each animal takes advantage of the instincts of others;—­ that the canon of natural history, ‘Natura non facit saltum,’ is applicable to instincts as well as to corporeal structure, and is plainly explicable on the foregoing views, but is otherwise inexplicable,—­all tend to corroborate the theory of natural selection” (p. 243).

We feel that it is the theory of evolution, or descent with modification, that is here corroborated, and that it is this which Mr. Darwin is mainly trying to establish; the sentence should have ended “all tend to corroborate the theory of descent with modification;” the substitution of “natural selection” for descent tends to make us think that these conceptions are identical.  That they are so regarded, or at any rate that it is the theory of descent in full which Mr. Darwin has in his mind, appears from the immediately succeeding paragraph, which begins “This theory,” and continues six lines lower, “For instance, we can understand, on the principle of inheritance, how it is that,” &c.

Again:-

“In the first place, it should always be borne in mind what sort of intermediate forms must, on my theory, formerly have existed” (p. 280).

“My theory” became “the theory” in 1869.  No reader who read in good faith could doubt that the theory of descent with modification was being here intended.

“It is just possible by my theory, that one of two living forms might have descended from the other; for instance, a horse from a tapir; but in this case direct intermediate links will have existed between them” (p. 281).