No. 530] TEMPERATURE ON GROWING MICE 93 



considerable changes in the temperature of the surround- 

 ing atmosphere. 5 



Assuming, provisionally, the truth of this proposition, 

 we may discount in advance the objection that the germ- 

 cells of a mammal may be influenced by differences of 

 temperature as' such. If these differences affect the 

 germ-cells at all, and it is reasonable to believe that they 

 may do so, they must act upon them indirectly. I shall 

 revert to this point again shortly. 



As some of you may perhaps already know, I have 

 succeeded for several years past in producing very 

 decided quantitative differences in certain of the external 

 parts of mice through the action of widely differing tem- 

 peratures. . . . (This part of the discussion has been 

 omitted in the printed report, since the results in ques- 

 tion have already been fully published.) 



In experiments such as those which I am describing, it. 

 is obviously impossible to subject a single individual to 

 both extremes of temperature during growth, and to com- 

 pare the differing effects of these upon structure. We 

 therefore, of necessity, resort to a comparison of aver- 

 ages, based upon as many individuals as possible. If 

 each of the contrasted groups is sufficiently large, and if 

 its members have been taken at random, the presumption 



