there is no ground for inferring its exhaustibility. The limited vitality
is an unproved and unprovable conjecture. The evolutive force, dying out in
the using, is either the same conjecture repeated, or a misapplied analogy.

After all--apart from speculative analogies--the only evidences we possess
which indicate a tendency in species to die out, are those to which Mr.
Darwin has called attention. These are, first, the observed deterioration
which results, at least in animals, from continued breeding in and in,
which may possibly be resolvable into cumulative heritable disease; and,
secondly, as already stated (p. 285), what may be termed the sedulous and
elaborate pains everywhere taken in Nature to prevent close
breeding--arrangements which are particularly prominent in plants, the
greater number of which bear hermaphrodite blossoms. The importance of this
may be inferred from the universality, variety, and practical perfection of
the arrangements which secure the end; and the inference may fairly be
drawn that this is the physiological import of sexes.
It follows from this that there is a tendency, seemingly inherent, in
species as in individuals, to die out; but that this tendency is
counteracted or checked by sexual wider breeding, which is, on the whole,
amply secured in Nature, and which in some way or other reenforces vitality
to such an extent as to warrant Darwin's inference that "some unknown great
good is derived from the union of individuals which have been kept distinct
for many generations." Whether this reenforcement is a complete preventive
of decrepitude in species, or only a palliative, is more than we can
determine. If the latter, then existing species and their derivatives must
perish in time, and the earth may be growing poorer in species, as M.
Naudin supposes, through mere senility. If the former, then the earth, if
not even growing richer, may be expected to hold its own, and extant species
or their derivatives should last as long as the physical world lasts and
affords favorable conditions. General analogies seem to favor the former