THE AMERICAN NATURALIST [Vol. XLIV 



The young were measured at the age of forty-two days. 

 It was found to be impracticable to make certain of these 

 measurements satisfactorily without subjecting the ani- 

 mals to ether. They were not, however, killed at this 

 time. The weight, and the length of tail, foot and ear 

 were determined. The linear measurements were taken 

 with a graduated sliding caliper, indicating tenths of a 

 millimeter. In the case of foot and ear, two measure- 

 ments were made of each, the mean figure being em- 

 ployed in the computations. 6 



Of these mice, 286 survived to the age of six weeks; 

 there being 141 offspring of warm-room parents, belong- 

 ing to 33 litters, and 145 offspring of cold-room parents, 

 belonging to 30 litters. From this earlier series of meas- 

 urements the following gross averages were obtained : 



Cold-room descendants 10.897 71.04 17.833 12.434 



Warm-room descendants ....10.631 71.19 17.960 12.536 



It will be seen at once that, although the offspring of 

 the warm-room mice average slightly less in weight, 

 they have slightly longer tails, feet and ears than the 

 offspring of the cold-room mice. These differences are 

 exactly such as were noted, on a larger scale, in the 

 parent generation. But such gross averages do not, in 

 themselves, mean very much. In each of the contrasted 

 groups are comprised individuals of widely different 

 size (the extremes were 6.5 and 19.3 grams). Our ma- 

 terial, therefore, is not at all homogeneous. 



Accordingly, I have divided the animals into groups, 

 comprising individuals of approximately the same 

 weight. 7 Herewith are presented in tabular form the 

 results of such an analysis. 



From the table it will be seen that there are eleven 

 groups in which a comparison between warm-room and 



•The average difference between the first and second reading of the 

 caliper was 0.19 mm. for the foot, and 0.12 mm. for the ear. 



*In my complete table I have likewise dealt with the sexes separately. 

 Lack of space prevents this procedure here. 



