176 ANIMAL BEHAVIOUR 



determined by its momentum and the circumstances, but 

 there is a spice of unpredictability in the ways of most 

 living creatures. The unexpected often happens. There are 

 indeed uniformities of sequence in the reactions of organisms, 

 otherwise no science of behaviour were possible, but there 

 is an undeniable appearance of free agency. It is inter- 

 esting to watch under the microscope the Brownian move- 

 ment seen when minute granules of sepia or the like are 

 jostled hither and thither, probably by the invisible ions 

 which abut against them, but the scene changes in character 

 when we put in a vigorous unicellular organism. It does 

 not 'take charge' like a gun torn from its attachment on 

 board ship; it commands its course. Only in the realm of 

 organisms is there true behaviour in which the creature 

 is an agent and exhibits a correlated or concatenated series 

 of acts, effective towards some definite result, favourable to 

 the continuance and harmony of vital processes. 



There are two master-activities in the animal organism 

 for the sake of which life is worth living movement and 

 feeling, contractility and irritability, the functions in most 

 cases of the muscular and nervous systems respectively. 

 These master-activities are kept agoing, the relevant struc- 

 tures are kept in working order, by the other everyday func- 

 tions of nutrition, circulation, respiration, excretion, and so 

 on ; never forgetting, in connection with Vertebrates at least, 

 the fundamental trigger-pulling and regulative function of 

 the organs of internal secretion. These everyday functions 

 are the pre-conditions of behaviour ; and growth and maturity 

 may also condition behaviour. But behaviour itself is much 

 more, it means that the organism is an agent and that it 

 exhibits a correlated or concatenated series of actions. 



