366 ELEMENTS OF BIOLOGY 



about some types of insect larvae which infest plants. That the 

 action of the insect is specific is shown by the fact that on the same 

 tree several types of galls may occur, each peculiar to some sort of 

 insect larva. A skilled entomologist may distinguish the type of 

 insect larva merely by observing the type of gall enclosing it. 



Individual Modifications That Are Adaptive. Individual 

 structural adaptations, as distinct from those inherited as species 

 characters, are modifications induced by the conditions of a par- 

 ticular environment; they represent, as it were, the attempt of the 

 body of each individual animal to adjust. Such adaptations include 

 the widest variety of structural changes from the normal and usual 

 shape, compensatory enlargement of muscles in one limb when the 

 other is injured or removed, the encasement of foreign bodies by 

 ".car tissue, and similar individual characters. There is no evidence 

 that such changes, imposed on the animal body from without, are 

 passed on to future generations, regardless of whether or not the 

 modifications enable the animal to cope more successfully with its 

 surroundings. Individual modifications that are adaptive are the 

 solutions of individual, not of species difficulties; each generation 

 must solve such problems anew when, or if they are encountered. 



The Nature of Response to Environment. Individual 

 functional adaptations represent the constant changes in the proto- 

 plasmic reaction system brought on by the constantly occurring 

 environmental changes. A fairly clear picture of the relation be- 

 tween the animal and its environment may be obtained by com- 

 paring the organism to a flowing stream. 



The streaming motion of the water is evidence of kinetic energy. 

 The path the water follows depends on the nature of the ground; 

 obstacles deflect it here, slopes encourage a more rapid flow there, 

 the force of the stream erodes away the bank at another place. So 

 the direction of the flow and the changes in direction, the features 

 that distinguish a river, are the result of the reaction between the 

 stream and its environment. It is not possible to conceive of a river 



