20 C. M. JACKSON 



of more recent biometric investigations in various lines by Pear- 

 son and others, it appears (cf. Encyclopaedia Britannica, ISth 

 edition, vol. 27, p. 911) that in general the fraternal S. D. = 



racial S. D. X — , reckoning the fraternal correlation at 0.50. 



According to this formula, the fraternal variability would amorunt 

 to about 87 per cent of the racial variability. Compared with 

 these figures the fraternal variability of the rat, which is less 

 than half of the racial, appears relatively small. This, however, 

 is intra-litter variability, which may be somewhat less than fra- 

 ternal variability in general. The latter could be determined for 

 the rat only by comparing numerous litters born from the same 

 pair of rats at different times. 



GROWTH AND VARIABILITY OF THE INDIVIDUAL ORGANS 



1. Head 



In a previous paper by Jackson and Lowrey ('12), the relative 

 growth of the head was considered. It was pointed out, as may 

 be observed in table 3, that the head (sexes together) of the albino 

 rat increases in relative size from an average of about 23 per 

 cent of the body at birth to a maximum of nearly 26 per cent 

 at seven days, after which it decreases gradually to about 10 

 per cent in the adult. The figures in the table for the percentage 

 at one year are probably somewhat high, as they include some 

 slightly emaciated individuals, in which the head is always rela- 

 tively large. In very large rats (above 300 grams in body weight) 

 the head continues to decrease in relative weight and may reach 

 8 per cent or less. 



As is furthermore evident from table 3, the variability of the 

 head is less than that of the body as a whole, the coefficient 

 increasing from about 9 in the newborn to about 13 in the adult. 

 The variation also appears slightly greater in the male than in 

 the female (except at six weeks), as is also the case with the 

 whole body. 



The coefficient of correlation between the head and the total 

 body weight varies at different ages from 0.76 to 0.95 (sexes 

 together). These figures are somewhat too high, however, the 



