150 SPECIAL FEATURES 
all. Islands which do not lie very near a continent, 
and are separated from the nearest continent by 
deep water, that is, do not le upon the same con- 
tinental shelf as the continent, show great general 
similarity as regards their fauna. In such islands there 
are no indigenous land mammals, often with the doubt- 
ful exception of mice and rats, animals readily intro- 
duced by man. Amphibians are also absent, a fact 
readily explained when we recollect that both the eggs 
and adults are very intolerant of salt, and therefore 
could not be readily transported across sea-water by 
floating logs of wood or any similar means. In such 
islands generally the commonest land forms are birds 
and invertebrates, especially insects and land shells. 
These in all cases tend to run into local races, due 
apparently to the isolation of the stock. Further, fly- 
ing forms tend to lose their wings or to possess only 
limited powers of flight. This is very marked in some 
birds, thus the flightless dodo, the solitaire, the kiwi of 
New Zealand, and so forth are all inhabitants of islands. 
It is noticeable that the power of flight tends specially 
to diminish in animals found in islands much exposed 
to hurricanes, for here even short flights might expose 
the animals to strong winds, which would sweep them 
out to sea, and thus destroy them, while those which 
flew little or not at all would be more likely to breed 
and continue their own type. 
Reptiles are not very frequent in islands separated 
by deep water from continents, and the presence of 
the large tortoises in the Galapagos Islands is held by 
some to indicate a connexion, at a remote period, with 
the continent of America. Wallace, on the other hand, 
believes that the ancestors of these forms were acci- 
dentally introduced, possibly on floating timber. 
