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The Origin and Classification of Islands. 
VERY island has its history, and in the case of all butnewly-formed 
volcanic islands or coral islets, every island has a double history, 
that of the island itself and that of its colonisation by the plants and 
animals which live uponit. The rocks of which an island consists 
will give us an insight into, though not always a complete knowledge 
of, its geological history; and a study of its living inhabitants will 
generally enable us to decide whether it has been colonised as an 
island or by direct former connection with a continent. Some biologists 
maintain that the fauna of an island will show whether it has ever 
been united to a continent or not, and this is the question which I 
propose to discuss in the present paper, because the answer to it 
involves some important inferences and conclusions. 
There are many ways in which an island can be formed ; it may 
be but a portion of a continent severed from the mainland by the 
erosive action of the sea; it may be-the mountainous part of a 
country which has sunk beneath the ocean ; it may have been thrown 
up from the floor of the ocean by volcanic action, or it may have been 
built up by the growth of reef-making corals. There are, however, 
only two ways in which an island can have been populated without 
the intervention of man; either it must once have been united toa 
continent and its inhabitants must be the descendants of those that 
then lived on that continent, or else its tenants must have been 
transported across the sea by the help of drift-wood, or by birds, or 
by winds and storms. 
It is evident, therefore, that 7 most cases there is likely to bea 
certain relation between the geological structure of anisland and the 
nature of its fauna and flora. Islands formed in the ocean, whether 
by direct upheaval, or by volcanic eruptions, or by coral growths, are 
not likely to possess a large assemblage of plants or of animals; they 
may be covered with vegetation, but the animals found on them must 
be the descendants of occasional waifs and strays. On the other hand, 
an island which has once been part of acontinent will, if it remain 
large enough, continue to support a large number of animals, and 
these will generally include a certain number of Mammalia and 
Amphibia. 
Islands have consequently been divided into two great classes— 
