erations arising
before the next upheaval, the sensible or insensible alterations thus
produced in each species would become organised--there would be a more
or less complete adaptation to the new conditions. The next upheaval
would superinduce further organic changes, implying wider divergences
from the primary forms; and so repeatedly.

But now let it be observed that the revolution thus resulting would not
be a substitution of a thousand more or less modified species for the
thousand original species; but in place of the thousand original species
there would arise several thousand species, or varieties, or changed
forms. Each species being distributed over an area of some extent, and
tending continually to colonise the new area exposed, its different
members would be subject to different sets of changes. Plants and
animals spreading towards the equator would not be affected in the same
way with others spreading from it. Those spreading towards the new
shores would undergo changes unlike the changes undergone by those
spreading into the mountains. Thus, each original race of organisms,
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
organisation.

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 high