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.
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