440 Variation and Correlation of Skuil Measurements 
the remaining characters are compared, the greatest difference is found 
in the length of the nasal bone (mean differences amount to more than 
2 per cent), the height of skull and width of zygoma come next (slightly 
over 1 per cent), while the smallest differences are found in the remaining 
characters (less than 1 per cent). From the relation shown above the 
writer inclines to consider the relative development of the nasal bone in 
the rat as one of the secondary sexual characters. 
6. From the above it is clear that the female skull can not be consid- 
ered as an undersized male skull, nor the male skull as an overgrown 
female skull, since there is at least one significant difference in the skulls 
of the two sexes; i. e., the length of the nasal bone. 
7. The female cranium on the other hand may be considered as an 
undersized male cranium, and vice versa, since the differences found from 
the three cranial measurements in the two sexes are too small to be 
significant. 
8. The relation between the coefficients of correlation and regression 
is linear. 
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