234 
ISLAND LIFE. 
[part II. 
direct and simple, and even when more complex are far easier to 
comprehend than those of continents ; and they exhibit besides 
certain influences on the forms of life and certain peculiarities of 
distribution which continents do not present, and whose study 
offers many points of interest. 
In islands we have the -facts of distribution often presented 
to us in their simplest forms, along with others which become 
gradually more and more complex ; and we are therefore able to 
proceed step by step in the solution of the problems they present. 
But as in studying these problems we have necessarily to take 
into account the relations of the insular and continental faunas, 
we also get some knowledge of the latter, and acquire besides so 
much command over the general principles which underlie all 
problems of distribution, that it is not too much to say that when 
we have mastered the difficulties presented by the peculiarities 
of island life we shall find it comparatively easy to deal with the 
more complex and less clearly defined problems of continental 
distribution. 
Classification of Islands with reference to Distribution . — Islands 
have had two distinct modes of origin ; they have either been 
separated from continents of which they are but detached frag- 
ments, or they have originated in the ocean and have never 
formed part of a continent or any large mass of land. This 
difference of origin is fundamental, and leads to a most 
important difference in their animal inhabitants; and we 
may therefore first distinguish the two classes — oceanic and 
continental islands. 
Mr. Darwin appears to have been the first writer who called 
attention to the number and importance, both from a geological 
and biological point of view, of oceanic islands. He showed that 
with very few exceptions all the remoter islands of the great 
oceans were of volcanic or coralline formation, and that none 
of them contained indigenous mammalia or amphibia. He also 
showed the connection of these two phenomena, and maintained 
that none of the islands so characterised had ever formed part of 
a continent. This was quite opposed to the opinions of the 
scientific men of the day, who almost all held the idea of 
continental extensions, and of oceanic islands being their 
