BIOLOGICAL IMPORTANCE OF MOTION 123 



the activities of the green cells of plants. Animals either 

 move to places where these products are or else cause the 

 products to come to them. As an example of the first 

 type may be mentioned any grazing animal which goes 

 where grass is and then eats it. As an example of the 

 second take the oyster which is anchored firmly to the 

 rock, and by movements of cilia on certain of its surfaces 

 within the shell causes a current of water to sweep 

 through its gill cavities in such a way that minute food 

 particles which the water may contain can be sifted out 

 and consumed. 



(2) Motion in the Struggle for Existence. — Where 

 animals are at all numerous competition enters, intro- 

 ducing the element of struggle, either of two animals 

 for the same plant food or of some animals to use other 

 animals for food. Here we find a second biological neces- 

 sity for motion, namely, the avoidance of injury. Mo- 

 tions for this purpose may take the form either of flight 

 or combat. Closely related to these are motions required 

 for making adjustments to climatic conditions. A search 

 for shelter from the elements or for warm quarters in 

 which to sleep are examples. 



(3) Motion and Perpetuation of the Race. — Next in 

 order comes a series of motions which are related, immedi- 

 ately or remotely, to the perpetuation of the race. The 

 pursuit of a mate, strife with competitive suitors, the 

 care of the young, all involve many motions and all have 

 immediate bearing on racial perpetuation. Of more re- 

 mote connection, yet undoubtedly belonging here, are the 

 motions by which the young of both sexes strive to make 

 themselves attractive. Among civilized men there are 

 many activities which have to do with racial perpetuation, 

 although the relation is often not obvious. For example, 

 the sacrifices of parents in order that their children may 

 be properly educated come under the head of care of the 

 young, just as truly as does the toil of a mother for her 

 infant's welfare. 



