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). Ifthe 
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. 
