THE UNIQUENESS OF THE INDIVIDUAL 



could happen; the problem is whether or not they do happen, 

 and the answer to this problem is at present open. 



(ii) The inheritance of learned behaviour differences in rats. 

 McDougall argued that a fair test of Lamarckian inheritance 

 should be one in which the acquired character difference repre- 

 sented the outcome of an active and (in the everyday sense) 

 'purposive"* response by the subject, and should be such that 

 the results were open to quantitative assessment. He therefore 

 studied the inheritance of the acquired ability of rats to 

 learn one of alternative methods of getting out of a water trap. 



The trap was a water bath with a central entrance ramp and 

 two exit ramps, one brightly illuminated and so wired as to 

 give a tetanizing shock, the other dim but not electrified. The 

 two exits were alternated to prevent the complication of the 

 experiments by the learning of left-handed or right-handed 

 habits of emergence. The rats came from the inbred stock of 

 the Wistar Institute, and were divided into three groups of 

 which two were bred from at random, or at least without avoid- 

 able selection. The three groups were {a) untrained controls; 

 (6) experimental rats that had been trained in the tank; and 

 (c) rats which had been through the tank tests but which, 

 instead of being bred from at random, were deliberately 

 selected for breeding from those which showed the worst per- 

 formances. The criterion of learning status was the number of 

 tests that had to be given before an individual scored twelve 

 correct choices of exit successively. 



The results of McDougalPs experiments, reported over a 

 period of years in the British Journal of Psychology (1927, 1930, 

 1938; McDougall and Rhine, 1934), were as follows. The tank- 

 trained rats of group (h) improved in performance from a score of 

 120 errors in the first generation to only 36 in the thirty- fourth 

 generation of non-selective inbreeding. Unfortunately, the rats 

 'negatively'' selected from the dullards of each generation 

 (group c) improved from performance scores of 215 to 43 over 



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