160 WORLD-POWER AND EVOLUTION 



type. The most notable differences were that when the length 

 of the tail and foot were compared with the length of the body, 

 the warm-room mice had relatively longer tails and feet. This 

 accords with the conditions among many tropical animals when 

 compared with related species in the north. Perhaps the large 

 feet of Negroes are due to similar conditions. Lengthening of the 

 ear appeared in many of the warm-room animals, but was by no 

 means so distinct as the lengthening of the tail and foot. In 

 general the length of the body was greater in the warm room than 

 in the cold, but this was not constant. Other characteristics, 

 such as weight and hairiness, seemed to have no direct relation 

 to temperature. Thus it appears that while the low temperature 

 produced modifications in the relative length of different parts of 

 the body, it did not produce what may be called adaptive modi- 

 fications. It is true that in general the rodents of warm countries 

 appear to have greater length of tail and foot than those of cold 

 countries, but there is no evidence that this is of any particular 

 value in the struggle for existence. Apparently mutations may 

 occur in any direction without respect to the causative agent. 

 It may happen that they are useful, as would be the case if the 

 hair were thicker in the coats of the cold-room mice. Under such 

 circumstances the mutation would be of value, and would pre- 

 sumably be preserved. 



When the mice that had been reared in the warm and cold rooms 

 were bred separately in the common room, their offspring dis- 

 played the same characteristics as their modified parents. This 

 happened even when a period of five months had elapsed since the 

 parents were removed from the cold and warm rooms and placed 

 in the common room. Therefore it seems to represent a real 

 inheritance. Moreover, the variations in the second generation 

 were more constant than in the first, so that the relative weight 

 and the length of the ear seemed to vary as constantly and regu- 

 larly as did the relative length of the tail and foot. This appears 

 in the following table based on 752 mice. The figures show the 



