VI DIFFICULTIES AND OBJECTIONS 145 



new species do not ai'ise. 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 Avould 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 Avould l)e very likely to 

 occur in a continuous area ; while, of course, any intercrossing 

 ^\^th 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, v/e have variation and adaptation on a much 

 larger scale ; and, whenever important physical changes de- 

 mand them, ^vith even greater rapidity. The far greater 

 complexity of the environment, together Anth the occurrence of 

 variations in constitution and habits, Avill often allow of 

 eflfective 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 



L 



