The Sixth Extinction: An Unnatural History

offers.
Natural selection eliminated the need for any sort of creative miracles.
Given enough time for “every variation, even the slightest” to
accumulate, new species would emerge from the old. Lyell this time was
not so quick to applaud his protégé’s work. He only grudgingly accepted
Darwin’s theory of “descent with modification,” so grudgingly that his
stance seems to have eventually ruined their friendship.
Darwin’s theory about how species originated doubled as a theory of
how they vanished. Extinction and evolution were to each other the warp
and weft of life’s fabric, or, if you prefer, two sides of the same coin. “The
appearance of new forms and the disappearance of old forms” were,
Darwin wrote, “bound together.” Driving both was the “struggle for
existence,” which rewarded the fit and eliminated the less so.
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.
Darwin used the analogy of domestic cattle. When a more vigorous or
productive variety was introduced, it quickly supplanted other breeds. In
Yorkshire, for example, he pointed out, “it is historically known that the
ancient black cattle were displaced by the long-horns,” and that these
were subsequently “swept away” by the short-horns, “as if by some
murderous pestilence.”
Darwin stressed the simplicity of his account. Natural selection was
such a powerful force that none other was needed. Along with miraculous
origins, world-altering catastrophes could be dispensed with. “The whole
subject of the extinction of species has been involved in the most
gratuitous mystery,” he wrote, implicitly mocking Cuvier.
From Darwin’s premises, an important prediction followed. If
extinction was driven by natural selection and only by natural selection,
the two processes had to proceed at roughly the same rate. If anything,
extinction had to occur more gradually.