ABSTRACT
This is the final report of an experiment of 20 years’ duration, in which we have repeated, in its essentials, the well-known experiment of William McDougall purporting to reveal a Lamarckian inheritance of the effects of training on rats. The test is one involving light discrimination, and McDougall recorded a steady improvement in the rate of learning on a succession of 32 generations; but he omitted to check the results against a properly conducted control.
Our experiment confirms McDougall to the extent that we too have obtained long duration trends of improvement in learning-rate (Figs. 2, 3); but we find that the effect is not sustained, and that it is, moreover, shown also by a control experiment, using animals of untrained ancestry. This forbids a Lamarckian interpretation.
Statistical analysis of the data indicates that the ‘condition’ of the rat markedly affects its speed of learning, and that progressive changes in learning-rate, over a succession of generations, are in reality correlated with the health of the laboratory colony, which is subject to periods of decline and recovery.
The last group necessarily contains only two generations (49 and 50) and in the first group of controls only generations 2-4 are, of course, included.
The main divergences from the pattern are the low values of the class mean in the May group, 1948-52, and the June group, 1932-37. In both groups the number of rats was small—considerably less than half the average number for the other months.
In the trained line our records show, as fertile, only those rats whose offspring were taken into training. When fertility was high, some first litters were discarded without their parentage being recorded and the fertility index would thus give an underestimate of the true level of fertility. This situation did not often arise, and, in fact, the value of the index for the trained line was not significantly different from its value for the controls, where all first litters were required, either for breeding or training.
