Darwin and Descent 187 
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 un- 
doubtedly 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 moditfica- 
tion, 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 con- 
clusion 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 comformable 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 
