28 ENVIRONMENTAL RELATIONS 



developed in, and never having been separated from, fluctuating con- 

 ditions, they do not respond to all environmental fluctuations. The 

 terms approximately constant and spontaneous used above are then 

 both relative. Any change in the external conditions sufficient to alter 

 the internal processes of the organism is called a stimulus. The visible 

 movement of the organism or other phenomena resulting from stimulation 

 is called the reaction. The reaction may be: (a) cessation of movement, 

 (b) initiation of movement, or (c) change in kind or direction of 

 movement. 



Fluctuations in the environmental conditions in nature usually 

 involve more than one factor. Experiments are necessary to determine 

 which factor is affecting the activities of the animal. The effect of the 

 various factors taken singly upon a few animals has been determined. 

 These factors are pressure, including currents and contact with other 

 bodies, shock, vibrations and sound, temperature, water, chemicals, light, 

 etc. For example, if we lower the temperature surrounding an insect 

 sufficiently, it will become apparently stiff and lifeless (35, p. 396). If the 

 temperature is raised again, the animal becomes active. The activity is 

 increased as the temperature is raised until a degree of heat nearly high 

 enough to kill it is reached, when the animal becomes inactive again. If 

 the temperature is raised only a little more, the animal dies. In general 

 changes in any factor produce either excitation or depression, or in 

 other words, an acceleration or retardation of the activities. In con- 

 nection with the acceleration or retardation of activity, animals fre- 

 quently turn toward or away from the source of light or sound, or in the 

 direction of a current of air or water. Or they congregate at a point 

 where the temperature or the light or the chemical conditions interfere 

 least with their internal processes. 



Such turnings or congregations are called tropisms or taxes (45). If 

 the animals turn toward or go toward the source of stimulation they are 

 said to be positive. If they turn away or go away or congregate at a 

 distance they are called negative. The names applied to the reactions 

 are given below. There are various theories as to the exact manner in 

 which these turnings and congregations are brought about, but, as a 

 rule, animals congregate where their internal processes are least inter- 

 fered with, and random movements nearly always play some part in 

 the process. There are two sets of terms applied to such responses 

 as described above; they are given in parallel columns below (p. 29). 

 Taxis means arrangement. Tropism means turning. 



