416 



PRACTICAL ZOOLOGY 



west toward the Atlantic, or east toward the Gulf of Bothnia, 

 as the case may be, regardless of how the valleys trend, climbing 

 a mountain instead of going around it, and, undeterred by any 

 river or lake, keep persistently onward until finally some sur- 

 vivors reach the sea, into which they plunge and perish." They 



are said to march in " par- 

 allel lines three feet apart" 

 and "gnaw through hay 

 and corn stacks rather 

 than go around." 



Geographical Distribu- 

 tion. The various species 

 of mammals and other 

 animals are rather defi- 

 nitely restricted to certain 

 regions on the earth's sur- 

 face. The earth has an 

 area of about two hundred 

 million square miles, five- 

 eighths of which is covered 

 by the sea. This vast ter- 

 ritory is not uniform, but 

 presents a great number of sets of conditions. The principal 

 habitats are the solid earth, the liquids upon the earth, and the 

 atmosphere. The facts of geographical distribution have led to 

 the formulation of the three following laws: (i) the law of 

 definite habitats, (2) the law of dispersion, and (3) the law of 

 barriers and highways. 



THE LAW OF DEFINITE HABITATS. Among the most impor- 

 tant physical factors that determine the habitat of an animal are 

 temperature, water, light, and food. The continent of North 

 America has been divided by scientists into definite regions, 

 according to the sum total of the temperature during the season 

 of growth; and regions of a certain temperature, though widely 

 separated, are liable to support similar kinds of animals. Winter 



FIG. 283. The Norwegian lemming. 

 (From Ingersoll.) 



