be overthrown by a change in the outer
actions or must undergo perturbations that cannot end until there is a
readjusted balance of functions and correlative adaptation of
structures. But where the external changes are either such as are fatal
when experienced by the individuals, or such as act on the individuals
in ways that do not affect the equilibrium of their functions, then the
readjustment results through the effects produced on the species as a
whole: there is indirect equilibration. By the preservation in
successive generations of those whose moving equilibria are less at
variance with the requirements, there is produced a changed equilibrium
completely in harmony with the requirements.

Even were this the whole of the evidence assignable for the belief that
organisms have been gradually evolved, Mr. Spencer holds that the belief
would have a warrant higher than is possessed by many beliefs which are
regarded as established. As a matter of fact, however, the evidence is
far from exhausted. At the outset of the first volume of "Principles of
Biology," it was remarked by the author that the phenomena presented by
the organic world as a whole cannot be properly dealt with apart from
the phenomena presented by each organism in the course of its growth,
development, and decay. The interpretation of either class of phenomena
implies interpretation of the other, since the two are in reality parts
of one process. Hence the validity of any hypothesis respecting the one
class of phenomena may be tested by its congruity with phenomena of the
other class. In the second volume of "The Principles of Biology," Mr.
Spencer passes to the more special phenomena of development, as
displayed in the structures and functions of individual organisms. If
the hypothesis that plants and animals have been progressively evolved
be true, it must furnish us with keys to these special phenomena. Mr.
Spencer finds that the hypothesis does this, and by doing it gives
numberless additional vouchers for its truth. It is impossible for us
here to review, even in outline, the extensive field traversed in the
second volume of "Principles of Biology." We would not omit, however,
to direct attention to the interesting conclusion reached by Mr. Spencer
toward the close of the volume with regard to the future of the human
race considered from the viewpoint of the possible pressure of
population upon subsistence. He points out that in man all the
equili