46 BASIS OF THE PEOBLEM 



which the nervous system has not been differentiated, it has been 

 suggested that the term ' autotype ' should be used.^ 



Keactions, however, probably never follow invariably upon 

 the repetition of the same stimulus. Among the lowest class 

 of animals, the Protozoa, a free-swimming ciliate Infusorian, 

 will, if it comes into contact with something in the medium 

 which is abnormal, it may be, for example, an alkaline solution, 

 stop and go backwards. This is an example of a reflex action. 

 An analysis of its subsequent movements shows that, if it again 

 and again meets the alkaline solution, some modification of 

 the reaction takes place. The animal may show increased 

 activity until either it is overcome by the disturbing medium 

 or it has freed itself from it. So again after the digestion of 

 food, such an organism shows a different response to the presence 

 of food. 



More definite instances of different reactions following upon 

 the same stimulus are provided by experiments of changing the 

 nature of the medium. An Infusorian will react violently against 

 certain new elements in the medium to which after a time it 

 becomes habituated, when the reaction no longer follows. Very 

 interesting experiments have shown that even among the simplest 

 class of organisms there is a certain learning from experience. 

 ' A Stentor (one of the ciliate Infusorians) if gently touched upon 

 one side will contract upon its stalk, but will soon open out again. 

 Touched once more, it will perhaps bend to one side, and if con- 

 tinually molested in this manner, it will uproot itself in pardon- 

 able dudgeon and swim away. That is to say it has several ways 

 of reacting to the stimulus and seeking equilibrium, and, if one 

 fails, it tries another. But now when it anchors itself again, it 

 seems to have learnt something, for if again touched it does not 

 go through the stages of contracting and of bending aside. It 

 keeps to its more radical remedy and moves off again.' ^ It is 

 doubtful whether there is any further complication of this type 

 of behaviour among organisms which do not attain to the kind of 

 behaviour that we have now to describe. 



7. Herbert Spencer, as is well known, defined instinct as com- 

 pound reflex action. Though this definition cannot, for various 



' Jennings, Behaviour of the Lower Organisms, p. 277. ^ Hobhouse, 



Development and Purpose, p. 62. In this and the following sections I am indebted 

 to Professor Hobhouse's review of the evolution of animal behaviour. 



