“He who believes that each being has been created as we now see it, must occasionally have felt surprise when he has met,” &c. (p. 185).
Here the argument evidently lies between descent and independent acts of creation. This appears from the paragraph immediately following, which begins, “He who believes in separate and innumerable acts of creation,” &c. We therefore understand descent to be the theory so frequently spoken of by Mr. Darwin as “my.”
“He who will go thus far, if he find on finishing this treatise that large bodies of facts, otherwise inexplicable, can be explained _by the theory of descent_, ought not to hesitate to go farther, and to admit that a structure even as perfect as an eagle’s eye might be formed _by natural selection_, although in this case he does not know any of the transitional grades” (p. 188).
The natural inference from this is that descent and natural selection are one and the same thing.
“If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, _my theory_ would absolutely break down. But I can find out no such case. No doubt many organs exist of which we do not know the transitional grades, more especially if we look to much-isolated species, round which, according to my _theory_, there has been much extinction” (p. 189).
This makes “my theory” to be “the theory that complex organs have arisen by numerous, successive, slight modifications;” that is to say, to be the theory of descent with modification. The first of the two “my theory’s” in the passage last quoted has been allowed to stand. The second became “the theory” in 1872. It is obvious, therefore, that “the theory” means “my theory;” it is not so obvious why the change should have been made at all, nor why the one “my theory” should have been taken and the other left, but I will return to this question.
“Although we must be extremely cautious in concluding that any organ could not possibly have been produced by small successive transitional gradations, yet, undoubtedly grave cases of difficulty occur, some of which will be discussed in my future work” (p. 192).
This, as usual, implies descent with modification to be the theory that Mr. Darwin is trying to make good.