forms and
from what we may anticipate with regard to the lowest organic ones. This
presumption is strengthened by the knowledge of the increasing difficulties
which beset any attempt to indefinitely intensify any race characteristics.
The obstacles to this indefinite intensification, as well as to certain
lines of variation in certain cases, appear to be not only external, but to
depend on internal causes or an internal cause. We have seen that Mr.
Darwin himself implicitly admits the principle of specific stability in
asserting the singular inflexibility of the organization of the goose. We
have also seen that it is not fair to conclude that all wild races can vary
as much as the most variable domestic ones. It has also been shown {127}
that there are grounds for believing in a tendency to reversion generally,
as it is distinctly present in certain instances. Also that specific
stability is confirmed by the physiological obstacles which oppose
themselves to any considerable or continued intermixture of species, while
no such barriers oppose themselves to the blending of varieties. All these
considerations taken together may fairly be considered as strengthening the
belief that specific manifestations are relatively stable. At the same time
the view advocated in this book does not depend upon, and is not identified
with, any such stability. All that the Author contends for is that specific
manifestation takes place along certain lines, and according to law, and
not in an exceedingly minute, indefinite, and fortuitous manner. Finally,
he cannot but feel justified, from all that has been brought forward, in
reiterating the opening assertion of this chapter that something is still
to be said for the view which maintains that species are stable, at least
in the intervals of their comparatively rapid successive {128}
manifestations.

* * * * *

CHAPTER VI.

SPECIES AND TIME.

Two relations of species to time.--No evidence of past existence of
minutely intermediate forms when such might be expected _a
priori_.--Bats, Pterodactyles, Dinosauria, and Birds.--Ichthyosauria,
Chelonia, and Anoura.--Horse ancestry.--Labyrinthodonts and
Trilobites.--Two subdivisions of the second relation of species to
time.--Sir William Thomson's views.---Probable period required for
ultimate specific evolution from primitive ancestral
forms.--Geometrical increase o