14 



THE AMERICAN NATURALIST [Vol. XLIV 



Another set of measurements was made with this same 

 lot of mice when they reached the age of three months. 

 By that time the numbers had been considerably reduced 

 by death. There were at the later date 114 of the cold- 

 room descendants, and only 84 of the warm-room descend- 

 ants. The survivors all appeared to be in good health, 

 however. 



Tn order to exclude the possible influence of suggestion 

 or unconscious bias in determining these rather delicate 

 caliper measurements, I adopted the plan of keeping 

 myself in ignorance as to the parentage of each mouse 

 until the latter had been measured. 13 



I shall not at present enter into as full an account of 

 these later measurements as of the first series. The 

 differences between the gross averages are even less to 

 be relied upon here than in the case of the earlier figures, 

 since the two contrasted lots differed much more in their 

 mean size. The warm-room descendants were some- 

 what the heavier of the two, having a mean weight of 

 19.45 grams, as compared with 18.56 grams for the other 

 lot. The body length was also somewhat greater for 

 the former (87.683 mm.) than for the latter (86.703 mm.). 

 For statistical purposes, the animals have been grouped 

 in two different ways: (1) according to weight, as was 

 done previously, and (2) according to body length. 14 

 The latter method of grouping seems a much fairer one 

 than the first, for it is probable that the length of the 

 appendages is correlated primarily with body length, and 

 only incidentally with weight. The single weight-groups, 

 it may be added, contained individuals which differed 

 from one another by as much as 4 or 5 mm. of body length. 



To consider the second of these methods first, the ani- 

 mals were divided into groups, within each of which the 



