THE DISTRIBUTION OF ANIMALS. 239 
Islands. The ocean is a barrier which land animals 
seldom cross. Because of this, islands far from con- 
tinents have both a fauna and a flora differing from that 
of the nearest mainland, and differing also from one 
another. Frogs and toads are not found on oceanic 
islands because salt water has prevented their migration 
to these places. Bats, birds, and butterflies may, 
however, overcome more easily the barrier of the sea, 
and make their homes on these islands. Some large 
islands, like New Zealand, show faunas curiously 
resembling the ancient faunas of Europe or America. 
Doubtless, in such cases, the barrier of the sea has 
preserved the ancient life by preventing the incoming 
of strong invaders. Because of facts like these island 
faunas are of great importance to Zodélogists. 
Isolated Tracts. Other regions of the earth may 
resemble islands in their animal life because they are 
surrounded by barriers which certain species cannot 
cross. Thus, the part of a river above a cataract will 
have aquatic animals different from those below the 
falls. 
Most arctic animals perish when brought to southern 
climates. Animals living in the lowlands of equatorial 
regions cannot live on the highlands. Thus heat and 
altitude may prove as effective barriers as the sea itself 
in isolating species. All over the world there are 
tracts characterized by the presence or absence of 
certain plants or animals. 
Wanderings. Whenever an animal wanders into an 
unfamiliar region, differing from its original habitat, we 
wish to know what causes operated to bring about the 
change of location, how the animal supports its life 
under the new conditions, whether it is likely to exter- 
minate or reduce the numbers of any of the original 
inhabitants of the locality, and, finally, whether its 
own structure will be gradually modified to adapt itself 
better to the new conditions of life. In regard to 
many cases of change of dwelling-place these questions 
