ORGANIC RESPONSE 243 



ate surroundings; the water, for instance, in which a 

 fish swims, including in it such factors as temperature, 

 light, chemical substances, pressure, etc. We have 

 seen that the most impressive characteristic of living 

 matter is its ceaseless flux and flow. This, however, 

 is not only true of such unstable things as living 

 organisms, but is also true, according to the story of 

 Geology, of mountains and continents. The only 

 permanent thing in the Universe, organic or inor- 

 ganic, is its eternal changefulness. We have seen, 

 too, that the existence of an organism or the existence 

 of an aggregate of organisms is dependent upon a 

 most delicate balance of innumerable "forces." 

 But these forces may act external to the organism as 

 well as internal ; that is, they may be of the environ- 

 ment. Any change in the environment may thus 

 produce a corresponding change in the balance of the 

 organism. Such an environmental change may be 

 called a stimulus. The readjustment in the organ- 

 ism due to such environmental change may be of 

 two kinds. It may be a simple alteration of relations 

 comparable to the crystallization of water with the 

 lowering of the temperature to the freezing point, 

 in which case we speak of it as a physical response. 

 On the other hand, it may involve the release of 

 energy stored up in the protoplasm, which manifests 

 itself in ways peculiar to and dependent upon the 

 organization of the latter. This second type of 

 response is called the organic response or reaction. 

 The stimulus in such a v case may be compared ^ith 

 the impact of the hammer which explodes the 



