Speaking elsewhere of intermediate forms, he remarks:—
"Opponents will say—'show them me.' I will answer yes, if you will show me every step between bulldog and greyhound."
Here we see that the case of domestic animals was already present in his mind as bearing on the production of natural species. The disappearance of intermediate forms naturally leads up to the subject of extinction, with which the next extract begins.
"It is a wonderful fact, horse, elephant, and mastodon, dying out about same time in such different quarters.
"Will Mr. Lyell say that some [same?] circumstance killed it over a tract from Spain to South America?—(Never).
"They die, without they change, like golden pippins; it is a GENERATION OF SPECIES like generation OF INDIVIDUALS.
"Why does individual die? To perpetuate certain peculiarities (therefore adaptation), and obliterate accidental varieties, and to accommodate itself to change (for, of course, change, even in varieties, is accommodation). Now this argument applies to species.
"If individual cannot propagate he has no issue—so with species.
"If SPECIES generate other SPECIES, their race is not utterly cut off:— like golden pippins, if produced by seed, go on—otherwise all die.
"The fossil horse generated, in South Africa, zebra—and continued—perished in America.