64 OCEANIC ISLANDS. 



Oceanic Islands. — We have now to distinguish be- 

 tween the terrestrial and serial, for the distribution of 

 terrestrial types is profoundly modified by the present and 

 past distribution of land surface. ^Erial types, on the 

 other hand, are not affected by comparatively large straits 

 or channels. 



This is well illustrated by the fauna of Oceanic Islands, 

 An oceanic island is an island which has been widely 

 separated from the mainland either from its very origin or 

 from a very remote date. Its fauna consists entirely of 

 immigrants from the adjacent mainland. Its truly terres- 

 trial fauna is usually small, consisting of small invertebrates, 

 reptiles, or mammals which may have effected the journey 

 in logs of wood or by other accidental means. On the 

 other hand its aerial fauna may be rich, for bats, birds 

 and insects can easily migrate across the water. 



The most remarkable feature is that these serial types, 

 especially in small and widely-isolated islands, show a 

 tendency to give up their aerial habits and become 

 terrestrial. Thus " wingless " birds and '* wingless " insects 

 are characteristic of oceanic islands. The explanation of 

 this will be clear after reading Chapter X., but we may only 

 indicate here that these wingless types are, in most instances, 

 assumed to be descended from winged ancestors, and that 

 the very wings which bore their ancestors to the island 

 would to them be a source of danger, their use involving 

 a risk of being blown out to sea. The entire absence 

 of terrestrial predatory forms removes one of the first 

 necessities for wings ; hence the loss of wings resolves 

 itself into an adaptation to a very peculiar environment. 



Discontinuous Distribution. — The consideration 

 of oceanic islands shows that there is no finality nor 

 permanency in the fauna of an area. There is the same 

 ceaseless change and succession of types as we find else- 

 where in nature. A particular species of animal will spread 

 slowly from one or more centres and reach a climax of 

 wide distribution, from which it will slowly recede till 

 extinction ensues. This extinction will not take place 

 in regular order, from the original centre outwards, but 

 will, in most instances, leave isolated remnants of the race 



