GEOGRAPHICAL DISTRIBUTION OF ANIMALS 1537 



body is divided into rings or segments, to the interior 

 of which the muscles are attached comprehending 

 all insects and worms; and radiated animals, in which 

 the organs of motion or sensation radiate from a com- 

 mon centre such as star-fish. 



The opposite sides of a hill-range, even in cases 

 where the climate is nearly identical, and the passage 

 from one slope to the other easy, will often exhibit 

 different conditions of animal life. Isothermal lines 

 mark with hardly less precision in the case of ani- 

 mals than of plants the range of particular families 

 and species, in the direction of latitude and elevation 

 alike. The fact that such is the case testifies strongly 

 to the force of those instincts with which all animals 

 are endowed, and by which their habits are regulated. 

 The powers of locomotion possessed by animals 

 might at first sight seem calculated to favor a wider 

 extension of geographical range than belongs to vege- 

 tables, and in the well-known instances of migratory 

 species (of which the swallow and other birds are 

 familiar examples) such is undoubtedly the case. 

 But even these migrations are confined within a well- 

 defined range, determined by conditions of climate, 

 and facility of obtaining the necessary food. Birds in 

 general are separated, as markedly as quadrupeds, in 

 respect of their habitat, or geographical range. This 

 is equally true, indeed, in regard to every one of the 

 great classes into which the animal world is divided. 

 Each zone of the ocean, both in latitude and in the 

 direction of depth, has its proper forms of life. 



To take an instance from land animals, the ele- 

 phant is confined by natural instinct within the belt 



