NEW TYPES AMONG ANIMALS 161 



average amount by which the warm-room mice exceeded the cold- 

 room mice when comparisons were made between animals having 

 the same length of body. 



Excess of Warm-Room Mice over Cold-Room Mice when Animals 

 WITH THE Same Length of Body Are Compared 



Every one of these numbers is positive. Therefore it seems 

 hard to avoid the conclusion that the parent mice were themselves 

 modified directly by the climatic conditions, and that they passed 

 these modifications on to their descendants. This does not mean 

 necessarily that there is any such thing as the inheritance of 

 acquired characteristics. It seems rather to mean that during 

 the very early life of an animal its germ plasm or reproductive 

 tissue is subject to changes in response to external environment. 

 This is suggested by the fact that when Sumner exposed adult 

 mice to low temperatures neither they nor their offspring were 

 affected. On the other hand, a small number of mice were born 

 of parents that had been kept in the cold room during the first 

 two weeks of their lives, but were then transferred to the warm 

 room. The offspring of these mice showed the characteristic 

 cold-room variations. Thus it looks as if the first two weeks were 

 the critical time. This is especially interesting in view of the 

 fact that during early life, as Sumner conclusively proves, the 

 blood temperature of the white mouse, as of all young mammals, 

 is not constant. It varies in response to the external air. In 

 the adult mouse, however, the variations in temperature are very 

 slight, and can be detected with certainty only when the animal 

 is subjected to very sudden and marked changes of temperature. 

 Thus it appears that when the animal is young and its body tem- 

 perature can be reduced to a low level, the germ plasm is influenced. 



