“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.
“_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.
“The manner in which single species and whole groups of species become extinct accords well with _the theory of natural selection_” (p. 322).
“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).
“Let us now look to the mutual affinities of extinct and living species. They all fall into one grand natural system; and this is at once explained _on the principle of descent_” (p. 329).
Putting the three preceding passages together, we naturally inferred that “the theory of natural selection” and “the principle of descent” were the same things. We knew Mr. Darwin claimed the first, and therefore unhesitatingly gave him the second at the same time.
“Let us see how far these several facts and inferences accord with _the theory of descent with modification_” (p. 331)