POSTNATAL GROWTH IN THE ALBINO RAT 11 



C showing the ratios in a few cases where the gross body weight 

 was observed in the same litters at successive ages. 



It is clear that there is considerable variation among individual 

 litters, and evidently observations upon a much larger number of 

 litters would be necessary to determine the typical growth rela- 

 tions by sexes. The available data seem in general to confirm 

 the conclusion of Donaldson ('06) that during the first two months 

 of postnatal life the growth of the body of the albino rat is more 

 vigorous in the female. In the majority of litters, however, the 

 average body weight of the females does not appear to reach 

 that of the niales, at least at the ages observed. 



2. Variability in body weight 



It is evident from the foregoing that there is in some respects 

 considerable difference between my data and those of Donaldson 

 on the growth of the body of the albino rat. The question 

 naturally arises as to the significance of this difference, as both 

 series are presumably normal (excepting the possible effect of 

 lung disease in the older rats) . This brings up the general ques- 

 tion as to the nature and extent of normal variation in the growth 

 of the body. It is known that this variation is greatly influenced 

 by external factors, such as the quantity and quality of food, 

 temperature and so forth. Slonaker ('12) has recently demon- 

 strated that the growth of the albino rat in bod}^ weight may 

 be greatly retarded by exercise, and that it is also much less with 

 vegetarian than with mixed diet. 



These factors, however, were nearly alike in the case of Donald- 

 son's rats and mine, though unavoidable differences in environ- 

 ment may account in part for their difference in growth. But 

 even when the environment is kept constant, there still remains 

 the variability due to intrinsic factors, which probably varies some- 

 what according to the ancestral strain from which the animals 

 were derived, as well as according to litter, sex and individual. 

 Pearson ('00, p. 473) states that ''The individual contains in 

 itself, owing to a bathmic (i.e., intrinsic) law of growth, a varia- 

 bility which is quite sensible, being 80 to 90 per cent of the 

 variability of the race." As before mentioned, it is therefore 



