474 DISTRIBUTION OF LAND ANIMALS 



forms, depend upon their own powers of locomotion. In- 

 sects, birds, and bats are actively flying creatures, and as 

 a result of their efforts may be distributed over wide 

 areas. Among the last named animals and a large num- 

 ber of lower forms, wind frequently plays an important 

 part as a distributing agency. There are numerous 

 records of birds and insects which have been captured 

 aboard ship far out at sea, and it is altogether probable 

 that many of the oceanic islands have been populated 

 by wind-swept stragglers. An even greater number of 

 animals, of widely different relationships, may be car- 

 ried by ice or floating vegetation. Mice, rats, squirrels, 

 insects, snails, worms, and a 'host of other species are 

 known in many instances to have been transported by 

 such means for long distances. Finally, there are large 

 numbers of animals which have been widely scattered 

 over the earth through the agency of man. In some 

 instances, where a species has been introduced, to com- 

 bat an insect pest for example, the results are highly 

 satisfactory, but the rabbit in Australia and the English 

 sparrow in America, or the Norway rat of the world at 

 large and the cockroach in America are introduced species 

 which have multiplied abundantly and become annoy- 

 ing, to say the least. 



Tangible Barriers. — It is a well known fact that 

 each species of animal occupies a definite area upon the 

 earth's surface, and it is equally true, as pointed out, 

 that many types when transported into a new territory 

 thrive even better than in their original homes. Since 

 every species broadens its range as far as it can, it is 

 evident that barriers tend to check its movements. These 

 barriers are very evident or tangible in some cases; in 

 others they are intangible, or not so apparent. Mountain 

 ranges, especially those whose summits are covered with 

 snow, are important tangible barriers which hinder the 

 wandering of many of the smaller animals. Deserts are 

 likewise obstacles to the progress of delicate organisms 



