“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). Here the two “my theories” have been altered, the first into “our theory,” and the second into “the theory,” both in 1869; but, as usual, the thing that remains with the reader is the theory of descent, and it remains morally and practically as much claimed when called “the theory”—as during the many years throughout which the more open “my” distinctly claimed it. Again:— “All the most eminent palæontologists, namely, Cuvier, Owen, Agassiz, Barrande, E. Forbes, &c., and all our greatest geologists, as Lyell, Murchison, Sedgwick, &c., have unanimously, often vehemently, maintained _the immutability of species_. . . . I feel how rash it is to differ from these great authorities . . . Those who think the natural geological record in any degree perfect, and who do not attach much weight to the facts and arguments of other kinds brought forward in this volume, will undoubtedly at once _reject my theory_” (p. 310). What is “my theory” here, if not that of the mutability of species, or the theory of descent with modification? “My theory” became “the theory” in 1869. Again:— “Let us now see whether the several facts and rules relating to the geological succession of organic beings, better accord with the common view of the immutability of species, or with that of their _slow and gradual modification_, _through descent and natural selection_” (p. 312). The words “natural selection” are indeed here, but they might as well be omitted for all the effect they produce. The argument is felt to be about the two opposed theories of descent, and independent creative efforts. Again:— “These several facts accord well with _my theory_” (p. 314). That “my theory” is the theory of descent is the conclusion most naturally drawn from the context. “My theory” became “our theory” in 1869. Again:— “This gradual increase in the number of the species of a group is strictly conformable _with my theory_; for the process of modification and the production of a number of allied forms must be slow and gradual, . . . like the branching of a great tree from a single stem, till the group becomes large” (p. 314). “My theory” became “the theory” in 1869. We took “my theory” to be the theory of descent; that Mr. Darwin treats this as synonymous with the theory of natural selection appears from the next paragraph, on the third line of which we read, “On _the theory of natural selection_ the extinction of old forms,” &c. Again:—