ms, would become the root from which diverged
several races differing more or less from it and from each other; and
while some of these might subsequently disappear, probably more than one
would survive in the next geologic period: the very dispersion itself
increasing the chances of survival. Not only would there be certain
modifications thus caused by change of physical conditions and food, but
also in some cases other modifications caused by change of habit. The
fauna of each island, peopling, step by step, the newly-raised tracts,
would eventually come in contact with the faunas of other islands; and
some members of these other faunas would be unlike any creatures before
seen. Herbivores meeting with new beasts of prey, would, in some cases,
be led into modes of defence or escape differing from those previously
used; and simultaneously the beasts of prey would modify their modes of
pursuit and attack. We know that when circumstances demand it, such
changes of habit _do_ take place in animals; and we know that if the new
habits become the dominant ones, they must eventually in some degree
alter the organization. Observe now, however, a further consequence.
There must arise not simply a tendency towards the differentiation of
each race of organisms into several races; but also a tendency to the
occasional production of a somewhat higher organism. Taken in the mass
these divergent varieties which have been caused by fresh physical
conditions and habits of life, will exhibit changes quite indefinite in
kind and degree; and changes that do not necessarily constitute an
advance. Probably in most cases the modified type will be neither more
nor less heterogeneous than the original one. In some cases the habits
of life adopted being simpler than before, a less heterogeneous
structure will result: there will be a retrogradation. But it _must_ now
and then occur, that some division of a species, falling into
circumstances which give it rather more complex experiences, and demand
actions somewhat more involved, will have certain of its organs further
differentiated in proportionately small degrees,--will become slightly
more heterogeneous. Thus, in the natural course of things, there will
from time to time arise an increased heterogeneity both of the Earth's
flora and fauna, and of individual races included in them. Omitting
detailed explanations, and allowing for the qualifications which cannot
here be specified, we think it is