174 EVOLUTION AND GENETICS 



perature, where each series was bred separately, 

 and measurements were made of the offspring when 

 the mice were full grown. It was found that the tail, 

 foot, and ear length were greater in the mice of 

 warm-room jDarentage than that of cold-room par- 

 entage but the difference was not so great as that 

 between their parents. 



This may be interpreted to mean that the smaller 

 increase shown by the tails of these mice of the sec- 

 ond generation from warm-room parents was due to 

 the intermediate temperature in which they were 

 reared, while their length, which was greater than 

 that of the mice of cold-room parentage, was in- 

 herited from the warm-room parents. But how? 

 Was it the effect of cold on the germ-cells, or did it 

 come from the longer tails of their parents? It is not 

 easy to imagine that the effect was due to the direct 

 influence of the cold on the germ-cells since mice are 

 warm blooded and maintain a nearly constant body 

 temperature when adult, and as young mice they 

 were kept warm in the nest and by the brooding of 

 their mothers. JMust we then conclude that the germ- 

 cells are so sensitiye to slight differences in the size 

 of the organs of the body that the effects are shown 

 in the next generation? If so, might we not expect 

 that all individual differences would reappear in the 

 characters of the offspring? 



But this question, at least, has now been settled by 

 Johannsen's brilliant analysis on the non-inheritance 



