EFFECTS OF INBEEEDING ON BODY WEIGHT 47 



animals attained an unusually large size. After the maximum 

 effect of the stimulus had passed there was a gradual decline 

 to more normal conditions of metabolism and a corresponding 

 decrease in the average size of the individuals. I see no reason 

 to assume that the hereditary factors concerned in growth were 

 influenced either by malnutrition during the early part of the 

 experiment, or by favorable nutrition in the later generations. 



Crampe ('83) and Hoskins ('16) have noted that the growth 

 of albino rats is influenced to a considerable extent by the time of 

 year in which the animals are born. Rats born in the winter 

 months are larger at a given age, live longer and are more vig- 

 orous than those born in the summer or autumn. The superiority 

 of the winter-born rats has been most marked in the various 

 breeding experiments that I have been carrying on for several 

 years with different strains of rats. It is not improbable that 

 the rats of the seventh inbred generation owed some part of 

 their vigorous growth to the fact that they were born in the 

 most favorable season of the year, the early winter months. 

 The environmental agency here concerned in stimulating growth 

 is either temperature or humidity, possibly both. A moderate 

 degree of cold is apparently more conducive to rapid and vigor- 

 ous growth in rats than is heat : the reverse is true for the mouse, 

 according to the investigations of Sumner ('09; '15). Extreme 

 temperature, either heat or cold, has a very unfavorable effect 

 on the rat, making the animals exceeding susceptible to the rat 

 scourge, pneumonia, which invariably proves fatal to an animal 

 of any age. 



In these experiments, as already stated, there was a very care- 

 ful selection of breeding stock from the seventh generation on. 

 Small, weak, inferior animals were eliminated before reaching 

 maturity, and only the largest and most vigorous animals were 

 allowed to perpetuate their kind. By this rigid selection it was 

 possible to keep the animals up to a high standard for body size 

 and to make the strain apparently immune to the injurious 

 effects that would probably have followed from random ma tings. 

 In other inbreeding experiments where selection of breeding ani- 

 mals was made on the basis of size and vigor there was no 



