THE GEOGRArHICAL DISTRIBUTION OF ANIMALS 369 



a noted case. This island is separated from the mainland by 

 a channel of water about two hundred and thirty miles in 

 width. On this island nearly all of the species of animals 

 are different from those in Africa. Moreover, the species 

 of birds differ from those only two hundred and thirty miles 

 away. 



Salt water is a firm barrier to fresh- water animals. Thus 

 the fresh-water animals living on an island not two hundred 

 yards from the continent would be effectually debarred 

 from swimming to that continent. On the other hand, 

 there are many fish living in salt water that are debarred 

 from entering fresh-water lakes. 



The third great class of barriers may be called climatic, 

 which will include the effects of temperature, moisture, and 

 dryness. 



There is a great difference between the animals of the 

 tropics and those of the arctic regions. A leopard with its 

 short hair would never venture into the arctic region. On 

 the other hand, a polar bear would be exceedingly uncom- 

 fortable in India. The parrots of Brazil would die in Canada. 

 The sloths, ant-eaters, and monkeys of the tropics would not 

 survive in the north. Neither would the foxes, sables, and 

 minks of Canada be able to live long in Central America. 



As to the effects of moisture and dryness recall the toad. 

 On account of excessive evaporation from its skin it demands 

 a moist atmosphere and could not survive on the deserts 

 of the Southwest. On the other hand, the Gila monster 

 would find it extremely distasteful to live in the valley of the 

 Mississippi. 



Notwithstanding all these barriers to the distribution of 

 animals, every species is unconsciously forced to spread over 

 larger areas or pass out of existence. For, in time, a species 



