VI 
DIFFICULTIES AND OBJECTIONS 
145 
new species do not arise. Complete isolation, as in an oceanic 
island, will no doubt enable natural selection to act more 
rapidly, for several reasons. In the first place, the absence of 
competition will for some time allow the new immigrants to 
increase rapidly till they reach the limits of subsistence. 
They will then struggle among themselves, and by survival of 
the fittest will quickly become adapted to the new conditions 
of their environment. Organs which they formerly needed, 
to defend themselves against, or to escape from, 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 lie very likely to 
occur in a continuous area ; while, of course, any intercrossing 
with the original unmodified stock would be absolutely pre¬ 
vented. 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, Avhenever important physical changes de¬ 
mand 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 
