296 ANIMAL LIFE 
157. Modes of distribution—The means and modes of mi- 
gration and distribution are obvious in the case of animals 
that can fly or swim or make long journeys on foot. An 
island can be visited and become peopled by birds from the 
nearest mainland. Fishes and marine mammals can travel 
from o¢ean to ocean. But many animals have no means 
of crossing watery barriers. ‘Oceanic islands, that have 
been formed de novo in mid-ocean and are not detached 
portions of pre-existing continents, are almost invariably 
free from such animals as are incapable of traversing the 
sea. If sufficiently distant from any continent, oceanic 
islands are generally without mammals, reptiles, and am- 
phibia, but have both birds and insects and certain other 
invertebrates which are transported to them by involuntary 
migration.” 
As suggested in the last sentence, migration may be 
passive or involuntary. For example, those minute ani- 
mals that can become dried up and yet retain the power 
of renewing their active life under favorable conditions are 
sometimes carried in the dried mud adhering to the feet of 
birds, and may thus become widely distributed. Parasites 
are carried by their hosts in all their wanderings. Some 
animals, as rats and mice, are carried by ships and railway 
trains and thus widely distributed. 
158. Fauna and faunal areas.—The term fauna is applied 
to the animals of any region considered collectively. Thus 
the fauna of Illinois comprises the entire list of animals 
found naturally in that State. It includes the aboriginal 
men, the black bear, the fox, and all its animal life down 
to the Ameba. The relation of the fauna of one region 
to that of another depends on the ease with which bar- 
riers may be crossed. Thus the fauna of Illinois differs 
little from that of Indiana or Jowa, because the State con- 
tains no barriers that animals may not readily pass. On 
the other hand, the fauna of California or Colorado differs 
materially from that of adjoining regions, because a moun- 
