it means that a species that is well adapted to its environment can survive
with a low degree of fertility, whereas a species that is not well adapted
to its environment requires a high degree of fertility in order to survive.
Mr. Pell considers that a "capacity for survival" is synonymous with
"nervous energy"; but, as our total knowledge of nervous energy is limited
to the fact that it is neither matter nor any known force, the change in
words does not mark a real advance in knowledge.

The second generalisation is that "the variation of the degree of animal
fertility in response to the direct action of the environment shall bear
an inverse proportion to the variation of the survival capacity under
that environment." [54] Here Mr. Pell and I part company. I have already
(Chapter III) disputed the causal connection between birth-rate and
death-rate which Mr. Pell here asserts. His generalisation is made by
assuming that birth-rates and death-rates rise and fall together: that
conditions which produce a high death-rate will also produce a high
birth-rate and that conditions which cause a low death-rate will also cause
a low birth-rate; that the increase or decline of a population is due to
the direct action of the environment; and finally that "the _actual_ degree
of fertility is decided by the direct action of the environment." [55] On
that last rock Mr. Pell's barque sinks. The mistake here is analogous to
the old Darwinian fallacy, abandoned by Huxley and by Romanes, that natural
selection is a creative cause of new species. Even if the hypothesis of
evolution--and it is merely a hypothesis--be accepted, the only view
warranted by reason is that variation of species and their actual degree of
fertility may be produced, not by the direct action of environment, but by
the _reaction_ of species to their environment--a very different story.

There is no statistical evidence to prove a uniform correspondence between
birth-rates and death-rates, and it is improbable that there should be