34 BIOLOGY 



a definite life-cycle ; that is, perform certain functions, 

 remain true to its type, and give rise to new individuals 

 by which it will be survived. Those of the second class 

 simply accelerate, retard, or modify the effects of the 

 first type, or they may act as the finger on the trigger ; 

 that is, they may initiate the first set. 



If we glance quite briefly at a few of the chief types 

 of external stimuli, they will be found to be : (1 ) mechani- 

 cal, i.e. when the organism is actually touched by some 

 external agent ; (2) chemical, where the stimulus is due 

 not to the material itself but to some of its chemical 

 properties ; (3) thermal, where the stimulus is some 

 variation in the temperature ; (4) electric, where the 

 stimulus is the influence of an electric current or shock ; 

 and (5) photic, the access of light or its absence. 



The reactions occasioned by these external stimuli 

 are known as tropisms, and we have such words as 

 Heliotropism, the response to light ; Chemotropism, the 

 response to chemical stimulation ; Thigmotropism, 

 the response to mechanical stimulation; and many 

 others. 



Now every vital process varies in its activity within 

 certain limits according to the duration, intensity, and 

 quality of the stimulus, and the processes go on most 

 satisfactorily when the stimuli are of a definite intensity, 

 so that we have for every living organism an optimum 

 intensity at which the best response is obtained, a 

 minimum below which no response is obtained, and a 

 maximum at which all response ceases. 



Another point which must not be forgotten is that 

 very rarely indeed does a stimulus act alone ; it is usually 

 accompanied by one or more other stimuli, which may 



