ECOLOGY 285 



as a squirrel seeks the trees where nuts may be found. But there is a 

 host of animals, low in the scale of development, which can hardly be 

 said to have a preference. They do have definite physiological require- 

 ments, about" which however they probably know nothing. They are 

 found in places where these requirements are satisfied, but they did not 

 seek those places. Sometimes, no doubt, they come upon these habitats 

 by chance. In many cases, however, simple animals are led to particular 

 localities through the inexorable operations of their own responses to 

 stimuli. These reactions are so important as to require further discus- 

 sion. 



Organisms, whether animals or plants, whatever their degree of or- 

 ganization, respond to various sorts of stimuli. These stimuli are light, 

 temperature, gravity, galvanic current, contact, chemical substances, and 

 so on. The character of the response to one or more stimuli is usually 

 definite for a given species but may differ for different species. Without 

 too much refinement of terms, any reaction of an animal in response to a 

 stimulus is called a tropism (derived from a Greek word meaning to turn) . 

 If the reaction is toward the stimulus it is said to be positive, if away it is 

 negative. Responses to stimuH are named in accordance with the kind 

 of stimulus applied. Thus a response to gravity is called geotropism; 

 to light, phototropism; to temperature, thermotropism; to electric current, 

 galvanotropism; to contact, ihigmotropism; to chemicals chemotropism; 

 to water, hydrotropism; and so the list might be extended. In place 

 of the word tropism the word taxis in combination with a word denoting 

 the character of the stimulus is sometimes used, thus, geotaxis, photo- 

 taxis, galvanotaxis, thigmotaxis, and so on. The word taxis is from a 

 Greek word meaning to arrange, hence geotaxis means a certain 

 arrangement of an organism's body with reference to stimulation by 

 gravity. The modifying words positive and negative are used as before 

 stated. 



Why an animal responds in a certain wa,y to a definite stimulus 

 has not been determined. The fact of the response, however, and the 

 character of the response are not matters of question since these facts 

 can be determined experimentally under controlled conditions. Given a 

 certain set of conditions, the organism reacts in a certain way, not because 

 it has the power of choice but because it cannot behave in any other way. 

 Within the organism is a responsive mechanism which operates only in a 

 certain manner. One animal turns toward the light while an individual 

 of another species turns from the light. The same animal may respond 

 differently to light, depending on the other conditions that prevail; 

 but under the same conditions the response is invariable. Each animal 

 responds in its particular way because it cannot do otherwise. 



Reactions of Lernaeopoda. — How these reactions determine the 

 relation of an organism to its habitat may be illustrated by a summary 



