DISTRIBUTION OF ANIMALS. 377 



changed as to form a new species of animals, differing 

 permanently and strikingly from their ancestors. It is 

 believed that this is the way in which many new species 

 of animals have been formed. 



372. The Means of Dispersal. One of the chief ways 

 in which animals are dispersed from any given point is 

 by means of their own powers of locomotion as they go 

 about in search of food and of mates, or to escape such 

 temporary discomforts as winter, drouth, famine, and the 

 like. Animals differ very much in their powers of loco- 

 motion and hence in readiness of distribution. Animals 

 that live in water may be carried about, particularly in 

 the egg or larval stages, by the waves and currents. In a 

 similar way, many land forms and minute pond animals 

 that can resist drying, either in the eggs or in the adult, 

 may be swept far and wide by the winds. Many small 

 forms are carried by other animals, as in the case of snails 

 that may be carried from pond to pond, either in egg or 

 adult, on the muddy feet of water birds. Parasites are 

 carried by their hosts. Man has carried many species of 

 animals over the surface of the earth in his ships and other 

 vehicles. Illustrating this are sparrows, rats, mice, 

 cockroaches and many other insects. 



373. Barriers to Dispersal. The chief barriers that 

 stand in the way of a general and universal distribution 

 of animals are: differences in temperature; drouth, as 

 is seen in deserts; water, in the case of land forms, and 

 land, for water forms; salt-water for fresh- water types, 

 and fresh- water for salt-water types; physical barriers, 

 such as mountains on land and falls in rivers. These 

 barriers act differently in different cases. Only wide 

 oceans or deserts or very lofty mountains are barriers to 



