exhaustion of specific vitality, or some equivalent internal cause. As
might be expected from the nature of the inquiry, the facts which bear upon
the question are far from decisive. If the fact that species in general
have not been interminable, but that one after another in long succession
has become extinct, would seem to warrant this conclusion, the persistence
through immense periods of no inconsiderable number of the lower forms of
vegetable and animal life, and of a few of the higher plants from the
Tertiary period to the present, tells even more directly for the limitless
existence of species. The disappearance is quite compatible with the latter
view; while the persistence of any species is hardly explicable upon any
other. So that, even under the common belief of the entire stability and
essential inflexibility of species, extinction is more likely to have been
accidental than predetermined, and the doctrine of inherent limitation is
unsupported by positive evidence.

On the other hand, it is an implication of the Darwinian doctrine that
species are essentially unlimited in existence. When they die out--as
sooner or later any species may--the verdict must be accidental death,
under stress of adverse circumstances, not exhaustion of vitality; and,
commonly, when the species seems to die out, it will rather have suffered
change. For the stock of vitality which enables it to vary and. survive in
changed forms under changed circumstances must be deemed sufficient for a
continued unchanged existence under unaltered conditions. And, indeed, the
advancement from simpler to more complex, which upon the theory must have
attended the diversification, would warrant or require the supposition of
increase instead of diminution of power from age to age.

The only case we call to mind which, under the Darwinian view, might be
interpreted as a dying out from inherent causes, is that of a species which
refuses to vary, and thus lacks the capacity of adaptation to altering