GENETIC TYPE AND THE ENDOCRINES 695 



of such different animals to be different in some fundamental 

 way. Such a divergence may have to do with a difference 

 in the fragility of the C-R. Since the dog has a far more 

 delicately poised central nervous system than the sheep or 

 goat it is probable that the C-R is also far more fragile and 

 sensitive to disturbances in physiological functions. The re- 

 cent work of Liddell, James and Anderson ( '34) also estab- 

 lishes the fact that the conditioned reflexes in the sheep and 

 goat are not disturbed or destroyed so easily as in the dog. 

 The C-R in the former animals was, for example, unaffected 

 in any noticeable way by the action of stimuli intended to 

 distract, but in the case of the dog, as clearly shown by 

 Pavlov and confirmed many times in the present experiments, 

 the C-R tends to be easily destroyed by any one of a great 

 variety of distracting sounds, sights or odors ("external 

 inhibition"). Exact and detailed analysis of such funda- 

 mental differences in behavior from the comparative point of 

 view offers a field for further profitable investigation. 



It must be pointed out that the criterion or standard by 

 which the effects of thyroid deprivation are judged is not 

 the same in the experiments of Valkov and of Liddell as in 

 the present study. Both workers compared the behavior of 

 a normal control animal with that of an operated animal 

 but in the present investigation each animal has served as 

 his own control. Behavior in the pre-operative period wa 

 directly compared with behavior in the post-operative period 

 in one and the same animal. In conditioned reflex experi- 

 ments on higher animals the former procedure often leads 

 to some confusion in the interpretation of the results. The 

 conditioned reflex method deals specifically with the individual 

 organism and its individual performances and achievements. 

 It is becoming increasingly apparent that the common idea 

 of a "control" must be reinterpreted when one deals with 

 the physiology of individual achievement. The behavioral 

 performances and achievements of one animal obviously can- 

 not be fully compared with that of another as a control. 

 Experience shows that the behavior of one animal may not 



