XII 
GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 
353 
entered by the route of Kamchatka and Alaska, where the 
climate, even now so much milder and more equable than on 
the north-east of America, might have been warm enough in 
late Pliocene times to have allowed the migration of these 
animals. In Asia they were driven southwards by the 
competition of numerous higher and more powerful forms, 
but have found a last resting-place in the swampy forests of 
the Malay region. 
What these Facts Prove. 
Now these two cases, of the marsupials and the tapirs, 
are in the highest degree instructive, because they show 
us that, without any hypothetical bridging of deep oceans, 
and with only such changes of sea and land as are indi¬ 
cated by the extent of the comparatively shallow seas 
surrounding and connecting the existing continents, we are 
able to account for the anomaly of allied forms occurring 
only in remote and widely separated areas. These examples 
really constitute crucial tests, because, of all classes of animals, 
mammalia are least able to surmount physical barriers. They 
are obviously unable to pass over wide arms of the sea, 
while the necessity for constant supplies of food and water 
renders sandy deserts or snow-clad plains equally impass¬ 
able. Then, again, the peculiar kinds of food on which 
alone many of them can subsist, and their liability to the 
attacks of other animals, put a further check upon their 
migrations. In these respects almost all other organisms 
have great advantages over mammals. Birds can often fly 
long distances, and can thus cross arms of the sea, deserts, or 
mountain ranges; insects not only fly, but are frequently 
carried great distances by gales of wind, as shown by the 
numerous cases of their visits to ships hundreds of miles from 
land. Reptiles, though slow of movement, have advantages in 
their greater capacity for enduring hunger or thirst, their power 
of resisting cold or drought in a state of torpidity, and they 
have also some facilities for migration across the sea by means 
of their eggs, which may be conveyed in crevices of timber or 
among masses of floating vegetable matter. And when we 
come to the vegetable kingdom, the means of transport are 
at their maximum, numbers of seeds having special adaptations 
2 A 
