1 64 COMPARATIVE PHYSIOLOGY 



It is the essence of scientific method that it deals only with 

 relations that are the result of external observation, that it aims 

 at expressing these relations in quantitative terms, and that 

 it employs no assumptions that could be eliminated without 

 affecting the verifiability of its conclusions. We have then 

 to ask whether it is possible by objective analysis to obtain any 

 further light on the mechanism by which the simultaneous 

 application of two classes of stimuli may enter into the result 

 of the simple operation of one of them on a subsequent 

 occasion. Associative phenomena probably play comparatively 

 little part in the lives of any animals outside the vertebrate 

 series. This special development is characteristic of mammals 

 in general ; and is the chief glory of man. Though we are 

 here concerned primarily with the lower organisms, some 

 account must be given of those properties which pre-eminently 

 distinguish mammals from those animals which we elect to 

 regard as " lower " than them, creatures which in any case 

 appear to have less complex and less flexible possibilities of 

 behaviour. 



In the objective analysis of associative phenomena an 

 immense advance has been made during the past two decades 

 through the work of Pavlov and his associates. Pavlov studied 

 salivary secretion in dogs. In the absence of the cortex, the 

 entry of food in the mouth is an efficient stimulus for reflex 

 salivary secretion. With the cortex intact, sight or smell of 

 food also evokes secretion. Further analysis led to a funda- 

 mental distinction being drawn between a type of reflex which 

 is only known in the animal with its higher cortex intact and 

 the reflexes which exist in both normal and decerebrate pre- 

 parations. In the intact animal a previously indifferent stimulus 

 applied at suitable intervals simultaneously with the applica- 

 tion of a stimulus which unconditionally evokes a particular 

 response, eventually acquires the capacity to evoke the response 

 unaccompanied by the " unconditioned " stimulus. A new 

 non-inherited reflex has been brought into being, known 

 as a conditioned reflex ; its previously ineffective agent 

 is known as the " conditioned " stimulus. This appears to 

 hold good for a large number of reflexes ; the salivary reflex 



