NEW TYPES AMONG ANIMALS 159 



almost equally early period. In this respect the toads resemble 

 all the insects for which exact data are available. 



Thus far we have been dealing with changes produced in the 

 offspring of cold-blooded animals. Let us now take up some 

 experiments with warm-blooded creatures whose response is pre- 

 sumably much the same as that of man. 



During the years 1906 to 1911 Sumner carried on a series of 

 experiments to test the effect of heat and cold upon white mice. 

 He divided his 2,300 animals into two groups which were kept 

 in separate rooms from the beginning of November to the end 

 of March. One room resembled an ordinary unheated attic. Its 

 mean temperature was about 39.5 °F and the mean relative 

 humidity approximately 75 per cent. The other room was heated, 

 and had a mean temperature of 72.5 °F and a relative humidity of 

 approximately 30 per cent. The rooms varied a good deal, how- 

 ever, especially the cool room which was subject to essentially 

 the same fluctuations as the outside air. During the remaining 

 seven months both sets of mice were transferred to a common 

 room which averaged a little cooler than the warm room, although 

 it became decidedly warmer in midsummer. 



On four occasions a second generation was raised from mice 

 that had been subjected to either the warm or cold rooms. The 

 total number of such mice for which measurements are available 

 is 879. It was impossible to raise a third generation, for the mice 

 were attacked by some disease or weakness which prevented repro- 

 duction. This happened to both the warm-room and cold-room 

 animals. It apparently had nothing to do with the temperature. 

 The warm-room animals suffered more than the others, however, 

 which suggests that their relation to temperature may be like 

 that of man. 



The mice which lived in the warm and cold rooms respectively 

 from their very birth showed distinct and systematic variations 

 when measured with great accuracy at various ages. The cold- 

 room animals seemed to depart most widely from the ancestral 



