om, enemies, being no longer
required, would be encumbrances to be got rid of, while the power of
appropriating and digesting new and varied food would rise in
importance. Thus we may explain the origin of so many flightless and
rather bulky birds in oceanic islands, as the dodo, the cassowary, and
the extinct moas. Again, while this process was going on, the complete
isolation would prevent its being checked by the immigration of new
competitors or enemies, which would be very likely to occur in a
continuous area; while, of course, any intercrossing with the original
unmodified stock would be absolutely prevented. If, now, before this
change has gone very far, the variety spreads into adjacent but rather
distant islands, the somewhat different conditions in each may lead to
the development of distinct forms constituting what are termed
representative species; and these we find in the separate islands of the
Galapagos, the West Indies, and other ancient groups of islands.

But such cases as these will only lead to the production of a few
peculiar species, descended from the original settlers which happened to
reach the islands; whereas, in wide areas, and in continents, we have
variation and adaptation on a much larger scale; and, whenever important
physical changes demand them, with even greater rapidity. The far
greater complexity of the environment, together with the occurrence of
variations in constitution and habits, will often allow of effective
isolation, even here, producing all the results of actual physical
isolation. As we have already explained, one of the most frequent modes
in which natural selection acts is, by adapting some individuals of a
species to a somewhat different mode of life, whereby they are able to
seize upon unappropriated places in nature, and in so doing they become
practically isolated from their parent form. Let us suppose, for
example, that one portion of a species usually living in forests ranges
into the open plains, and finding abundance of food remains there
permanently. So long as the struggle for existence is not exceptionally
severe, these two portions of the species may remain almost unchanged;
but suppose some fresh enemies are attracted to the plains by the
presence of these new immigrants, then variation and natural selection
would lead to the preservation of those individuals best able to cope
with the difficulty, and thus the open country form would become
modified into a m