ACQUIRED CHARACTERS 173 



dence of the inherited effect of the enviroiiiiieiit is 

 open to the same objection — the inheritance of color 

 changes in hzards, the change in the breeding habits 

 of the midwife toad, and the development of horny 

 pads on the thumbs of the male. That the environ- 

 ment causes changes in some of these characters 

 need not be questioned, but that the effects produced 

 are transmitted to the next generation, through the 

 bodily changes produced, may be questioned, both 

 because of the inadequacy of the evidence and also 

 because in other cases where the materials are suit- 

 able for making such tests there is no evidence that 

 such influences produce such results. Perhaps the 

 most careful and thoughtful piece of analytical 

 work that has been done in this field is that by 

 Sumner, extending over five years, on the effect of 

 heat and cold on the length of the tail, ears, and feet 

 of white mice, as well as on the increase in the thick- 

 ness of the hair in the cold. 



Some of the mice were reared from birth in a cold 

 room, others in a warm room. The average difference 

 in temperature was eighteen degrees centigrade. 

 The tails of the mice in the warm room series were 

 longer than the tails of those in the cold, for mice of 

 the same body length. The length of the feet and of 

 the ears was also greater in the warmer room, al- 

 thouerh the effect of the cold on the ears was incon- 

 stant. These two kinds of mice were then brouglit 

 together in a common room of intermediate tem- 



