64 OCEANIC ISLANDS. 
Oceanic Islands.—We have now to distinguish be- 
tween the terrestrial and erial, for the distribution of 
terrestrial types is profoundly modified by the present and 
past distribution of land surface. Airial types, on the 
other hand, are not affected by comparatively large straits 
or channels. 
This is well illustrated by the fauna of Oceanic [slands. 
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 zerial 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 erial 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 
