46 Relation of Animal Motion to Animal Evolution. | January, 
It cannot be denied that organisms which are incapable of 
-= moving from place to place in search of food, or of migration to 
escape vicissitudes of temperature, are much more completely 
subject to the influences of their environment, than those that are 
capable of 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 defence 
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 migra- 
tions have escaped. When an irruption of destructive animal 
enemies has threatened an animal population with death, those 
members of it whose strength or speed ensured them safety, were 
the survivors. 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 de- _ 
pendent on temperature, those portions of the organism which 
furnish means of resistence to climatic vicissitudes have the 
deepest significance in the life history of any division of animals. 
The organs of circulation and motion are generally recognized 
as primary in the classification of Vertebrata. All situations where 
animal life is permitted by climate, support vegetable life also; $0 — 
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 N 
in the Sabordinate directions indicated by the teeth and beak. A : 
But here also organs of movement are of great importance; 5° 
that the herbivorous and carnivorous types at least, have ever in 1 
land animals (reptiles, birds and mammals) been characterized by — 
the structure of their feet also. q 
