CHAPTER VII 
SPECIAL FEATURES OF ISLAND FAUNAS 
ISLANDS present some interesting features as regards 
their animal life which makes them worthy of special 
study. We have already seen that mountain areas have 
special types of animals, because they form isolated 
regions cut off by the physical conditions from the 
neighbouring districts. The special conditions in their 
case include both climate and topography, and in what- 
ever region of the globe the mountains occur, if they are 
greatly elevated, the climatic conditions are similar. 
Islands are regions cut off from neighbouring regions 
by the sea, which forms a barrier to the passage of all 
but the marine and the flying animals. Even to flying 
animals a large expanse of sea may constitute a for- 
midable barrier, save in the case of those with excep- 
tional powers of flight. The consequence is that island 
faunas form a whole, and can be studied with much 
more ease than the animals of any other natural region. 
. The question whether a particular animal does or does 
not occur in an island is merely a question of observa- 
tion ; while that as to whether a form found on a 
‘mountain does or does not constitute part of the moun- 
tain fauna is a much more delicate one, involving an 
elaborate process of reasoning. In the one case a 
gradual migration with concurrent adaptation is pos- 
sible from plain to mountain or vice versa, but no such 
slow migration is possible in the case of islands. 
On the other hand, it is to be noted that while 
