Summary
In order to be in a position to examine the conclusions which McDougall has reached, 18 generations of rats have been trained. The figures provided by 1445 experimentais and 1014 controls are compared with those which McDougall derived from 21 generations of rats of the same origin.
Criticism is levelled at the methods adopted by McDougall for presenting his figures, and especially at his lack of adequate control.
In the case of my rats the average number of errors per rat made by individuals of the tank-trained stock has not decreased with the passing of the generations, and there is no difference whatsoever between the scores of the experimental and control stocks. I have encountered no evidence which would suggest that rats of the trained and control stocks respectively can be distinguished one from the other by differences in behaviour. This being so I cannot accept the results which McDougall obtained as being in themselves strong enough to carry the interpretation that he has placed upon them.
Analysis of my own pedigrees shows definitely that genetic factors are heavily concerned in the establishment of the scores. Two main classes of rats are involved, quick and slow, and in a general way quickness behaves as a dominant, slowness as a recessive. The parent-offspring correlation is 0.3. A “quick” strain has been developed as the result of consistent favourable selection. To “fix” a slow strain has proved to be a much more difficult task. The reasons for this are discussed.
I submit that there is no need to postulate, in explanation of the fact that the average scores of the earlier and later generations of McDougall’s rats differ, that some new quality has been acquired and is being transmitted, for the average score of a generation is determined by the proportion of quick and slow rats within it, and these proportions can, within limits, be prearranged.
Among my rats there is a great excess of those which tend to leave the tank habitually by one route during the first phase of their training when the light is constant and equal on both sides of the tank and when the platforms are not alive. A considerable number of rats reacted to light as light in the second phase of their training when the light was alternating but the platforms were not alive. Actually 29 experimental and 10 control rats “learnt” without receiving a single shock.
The relation of handedness and photophobia to the score is discussed.
I do not propose to carry the main study further. The search for the genetic basis of handedness and of photophobia is being continued.
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Crew, F.A.E. A repetition oe modougall’s lamarckian experiment. Journ. of Genetics 33, 61–102 (1936). https://doi.org/10.1007/BF03027604
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DOI: https://doi.org/10.1007/BF03027604