THE ORGANISM AND ITS ENVIRONMENT 375 



thermal animals, having mechanisms that render them more or less 

 independent of the temperature of the surroundings, have a greater 

 latitude of movement. Because of this fact and of his ability to 

 control his immediate environmental temperature to some extent, 

 Man is well distributed over a wide range of temperature and cli- 

 matic conditions. 



Chemotropism. Responses to chemicals are perhaps the most 

 common and continuous of all tropisms and constitute a most im- 

 portant part of animal behavior, from the lowest to the highest 

 free-living animals. To provide more effectively for the reception 

 of chemical stimuli the receptor organs of taste and smell (p. 224) 

 occur in a variety of types. Responses are both positive and negative. 

 If one walks over the mud flats along the seashore at low tide one 

 may find many small snails, for example Nasa obsoleta, in depres- 

 sions and along the trickling streams that follow the receding water. 

 If a clam is crushed and some of its fluids washed into one of these 

 streams, very soon the snails farther down become agitated and 

 turn with surprising speed and promptness and move up stream 

 toward the source of the clam juices. Bees may be conditioned ex- 

 perimentally to respond positively to certain odors and doubtless 

 their visits to flowers are in part responses of the same sort. On the 

 other hand, the fact is well known that all animals, including Man, 

 avoid environments which contain a concentration of offensive or 

 injurious chemicals. 



A special type of chemotropism is the positive response of plants, 

 particularly the roots of plants, to water. This fact is so commonly 

 observed that it requires no especial illustrations here. Again, we 

 do not have at hand a physico-chemical explanation of the mech- 

 anism which results in the very distinct positive growth of roots 

 toward water in the soil. This response is often spoken of as 



HYDROTROPISM. 



Other Responses and Behavior. In addition to these more 

 or less general tropisms, animals that are equipped with sound- 



