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.
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“The theory of natural selection is grounded on the belief that each new variety and ultimately each new species, is produced and maintained by having some advantage over those with which it comes into competition; and the consequent extinction of less favoured forms almost inevitably follows” (p. 320). Sense and consistency cannot be made of this passage. Substitute “The theory of the preservation of favoured races in the struggle for life” for “The theory of natural selection” (to do this is only taking Mr. Darwin’s own synonym for natural selection) and see what the passage comes to. “The preservation of favoured races” is not a theory, it is a commonly observed fact; it is not “grounded on the belief that each new variety,” &c., it is one of the ultimate and most elementary principles in the world of life. When we try to take the passage seriously and think it out, we soon give it up, and pass on, substituting “the theory of descent” for “the theory of natural selection,” and concluding that in some way these two things must be identical.
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“The manner in which single species and whole groups of species become extinct accords well with the theory of natural selection” (p. 322).
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“This great fact of the parallel succession of the forms of life throughout the world, is explicable on the theory of natural selection” (p. 325).
Again:—