152 HERBERT SPENCER

IV. Arguments from Distribution.—" Given that
pressure which species exercise on one another, in
consequence of the universal overfilling of their re-
spective habitats—given the resulting tendency to
thrust themselves into one another's areas, and
media, and modes of life, along such lines of least re-
sistance as from time to time are found—given besides
the changes in modes of life, hence arising, those
other changes which physical alterations of habitats
necessitate—given the structural modifications directly
or indirectly produced in organisms by modified con-
ditions 5 and the facts of distribution in space and
time are accounted for. That divergence and re-
divergence of organic forms, which we saw to be
shadowed forth by the truths of classification and the
truths of embryology, we see to be also shadowed
forth by the truths of distribution. If that aptitude to
multiply, to spread, to separate, and to differentiate,
which the human races have in all times shown, be a
tendency common to races in general, as we have ample
reason to assume 5 then there will result those kinds
of spacial relations and chronological relations among
the species, and genera, and orders, peopling the
Earth's surface, which we find exist. The remarkable
identities of type discovered between organisms in-
habitating one medium, and strangely modified
organisms inhabiting another medium, are at the same
time rendered comprehensible. And the appearances
and disappearances of species which the geological
record shows us, as well as the connections between
successive groups of species from early eras down
to our own, cease to be inexplicable" (Principles of
Biology, \. p. 489).ression." of scientific and trans-s intrinsic incoherence; worthless as