We must be cautious
In concluding that the most different habits of all could not graduate into each other; that a bat, for instance, could not have been formed by natural selection from an animal which at first could only glide through the air.—p. 204.
Again:—
I see no difficulty in supposing that such links formerly existed, and that each had been formed by the same steps as in the case of the less perfectly gliding squirrels, and that each grade of structure was useful to its possessor. Nor can I see any insuperable difficulty in further believing it possible that the membrane-connected fingers and forearm of the galeopithecus might be greatly lengthened by natural selection, and this, as far as the organs of flight are concerned, would convert it into a bat.—p. 181.
For instance, a swim-bladder has apparently been converted into an air-breathing lung.—p. 181.
And again:—
The electric organs of fishes offer another case of special difficulty: It is impossible to conceive by what steps these wondrous organs have been produced; but, as Owen and others have remarked, their intimate structure closely resembles that of common muscle; and as it has lately been shown that rays have an organ closely analogous to the electric apparatus, and yet do not, as Matteucci asserts, discharge any electricity, we must own that we are far too ignorant to argue that no transition of any kind is possible.—pp. 192-3.
Sometimes Mr. Darwin seems for a moment to recoil himself from this extravagant liberty of speculation, as when he says, concerning the eye,—
To suppose that the eye, with its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.—p. 186.
But he soon returns to his new wantonness of conjecture, and, without the shadow of a fact, contents himself with saying that—