Here “the belief in the transmutation of species,” or descent with modification, is treated as synonymous with “the theory of descent with slow modification through natural selection; “but it has nowhere been explained that there are two widely different “theories of descent with slow modification through natural selection,” the one of which may be true enough for all practical purposes, while the other is seen to be absurd as soon as it is examined closely.  The theory of descent with modification is not properly convertible with either of these two views, for descent with modification deals with the question whether species are transmutable or no, and dispute as to the respective merits of the two natural selections deals with the question how it comes to be transmuted; nevertheless, the words “the theory of descent with slow modification through the ordinary course of things” (which is what “descent with modification through natural selection” comes to) may be considered as expressing the facts with practical accuracy, if the ordinary course of nature is supposed to be that modification is mainly consequent on the discharge of some correlated function, and that modification, if favourable, will tend to accumulate so long as the given function continues important to the wellbeing of the organism; the words, however, have no correspondence with reality if they are supposed to imply that variations which are mainly matters of pure chance and unconnected in any way with function will accumulate and result in specific difference, no matter how much each one of them may be preserved in the generation in which it appears.  In the one case, therefore, the expression natural selection may be loosely used as a synonym for descent with modification, and in the other it may not.  Unfortunately with Mr. Charles Darwin the variations are mainly accidental.  The words “through natural selection,” therefore, in the passage last quoted carry no weight, for it is the wrong natural selection that is, or ought to be, intended; practically, however, they derived a weight from Mr. Darwin’s name to which they had no title of their own, and we understood that “the theory of descent with slow modification” through the kind of natural selection ostensibly intended by Mr. Darwin was a quasi-synonymous expression for the transmutation of species.  We understood—­so far as we understood anything beyond that we were to believe in descent with modification—­that natural selection was Mr. Darwin’s theory; we therefore concluded, since Mr. Darwin seemed to say so, that the theory of the transmutation of species generally was so also.  At any rate we felt as regards the passage last quoted that the theory of descent with modification was the point of attack and defence, and we supposed it to be the theory so often referred to by Mr. Darwin as “my.”

Again:-

“Some of the most ancient Silurian animals, as the Nautilus, Lingula, &c., do not differ much from the living species; and it cannot on my theory be supposed that these old species were the progenitors,” &c. (p. 306) . . .  “Consequently if my theory be true, it is indisputable,” &c. (p. 307).