356 ' MECHANICAL EVOLUTION. 



ject to the influences of their environment than those that are ca- 

 l)able cf such movement. Hence animals are much more inde- 

 pendent of the supply of food and of temperature than are plants. 

 Hence also, other things being equal, the greater the powers of 

 motion, the greater the independence. 



Powers of movement then enable animals to avoid extremes 

 of climate by migrations or by protective arts. They enable them 

 to procure food by making journeys in search of it, and by all 

 methods of capturing it. They furnish the agent of active defense 

 against enemies, and of successfully reproducing their kind. 



When, through changes of level of the earth's surface, drought 

 has overtaken a region, animals capable of the necessary migrations 

 have escaped. When an irruption of destructive animal enemies 

 has threatened an animal population with death, those members 

 of it whose strength or speed insured them safety, were the sur- 

 vivors. When land has been encroached upon by water to such a 

 degree as to bring starvation on its animal inhabitants, those which 

 could fly or swim have sought new localities. 



Since all food-supply, as well as the ability to obtain food, is 

 dependent on temperature, those portions of the organism which 

 furnish means of resistance to climatic vicissitudes have the 

 deepest significance in the life-history of any division of ani- 

 mals. 



The organs of circulation and motion are generally recognized 

 as primary in the classification of Vertehrata. All situations where 

 animal life is permitted by climate, support vegetable life also ; so 

 each of the primary divisions of animals presents types adapted to 

 the use of all kinds of food ; herbivorous, omnivorous, and carniv- 

 orous. Accordingly, it has been found that dental and other 

 structures connected with digestion define divisions of secondary 

 value and minor extent. Paleontology shows that the origin of 

 such divisions is of later date than that of the great classes first 

 mentioned ; and each of the latter has in its day been modified in 

 the subordinate directions indicated bv the teeth and beak. But 

 here also organs of movement are of great importance ; so that the 

 herbivorous and carnivorous types at least, have ever in land ani- 

 mals (reptiles, birds and mammals) been characterized by the 

 structure of their feet also. 



