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 ocean 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 Amcela. 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 Iowa, 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- 



