94 HENRY H. DONALDSON 
This table was formed by using the same body weights as are 
given in table 19, but in place of the records for cranial capacity, 
entering those for brain weight. The average percentage differ- 
ence in the brain weight is seen to be 14.4 per cent or 2.2 points 
higher than the corresponding value for the cranial capacity— 
which is 12.2 per cent. 
We have already found that according to the data in table 17, 
the crania of the European Norways have shrunk 2.1 per cent more 
than those of the Philadelphia Norways, and those of the Euro- 
pean Albinos 1.6 per cent less than those of the Philadelphia 
Albinos. Therefore if the average observed difference in the 
cranial capacity (12.2 per cent) were corrected for the excessive 
shrinkage of the European Norway crania, and the deficient 
shrinkage of the European Albino crania (1.e., 2.1 + 1.6 = + 3.7 
per cent) it would give a value of (12.2 + 3.7) 15.9 per cent as 
that for the anticipated average difference in brain weight. 
From table 20 the observed difference is seen to be 14.4 per 
cent. It is to be remembered however that this last is based on 
the brain weights of the Philadelphia forms and that we have 
already found, table 16, the brain weight of the European Nor- 
ways (Paris and London series) to be 1.2 per cent greater than 
that of the Philadelphia Norways. If we use this as a correction, 
then the average difference in brain weights becomes (14.4 + 1.2 
per cent) 15.6 per cent, or very close to the difference found (15.9 
per cent) when the cranial capacities are corrected for the varying 
amounts of shrinkage shown by the several series. 
Neither the nature of the data nor the method of comparison 
will justify us in pushing this argument in detail, but the general 
relations thus determined, clearly support the view that the dif- 
ference in cranial capacity here found—when corrected for the 
unequal shrinkage of the crania in the two forms of the Kuropean 
rats, and for the slight excess in the brain weight of the Huropean 
Norways, approximates the difference in brain weights which we 
should expect to find between the two European forms if the 
European and American rats were nearly alike in the relative 
weight of their central nervous systems. 
