564 SHINKISHI HATAI 
Castle, ’11) as an indication of the segregation of the character 
without dominance. My own data are not sufficient for the 
full discussion of this question and furthermore the material 
itself is not ideal for the purpose, owing to the probable influence 
of domestication on the Norway factor. However, so far as my 
present data show, the degree of variability in the first and second 
filial generations is practically identical. We find the coefficient 
of variation of brain weight for F,; to be 8.603 + 0.396 while that 
for F, is 8.774 += 0.348. 
It is desirable in this connection to state clearly the difficul- 
ties which at present prevent us from reaching a definite 
conclusion. These difficulties are the following: 
a) The wild Norway rats when kept under captivity are 
furnished with a presumably unaccustomed diet. Since in many 
animals domestication apparently alters the normal relation 
between the body size and brain weight (see Darwin, ’83 and 
Lapicque and Girard, ’07) the probable effect of captivity on the 
Norway rats as to their brain weights should be taken into con- 
-sideration. If the Norways were thus losing in relative brain 
weight the influence of the Norway factor might be diminished. 
b) The brain weight varies with the body size in the rats 
(see Donaldson, ’08, and Donaldson and Hatai, ’11). It is 
therefore possible that any factors which alter the normal growth 
of the body size may also alter the relative brain weights or even 
the brain weights may suffer quite independently of the body 
Size. 
In spite of these possible sources of error it is to be noted first 
that the mean brain weights in the F, and F, generations are 
well above those for the albino parents, showing that some 
factor was at work to raise the brain weight above that char- 
acteristic for the Albinos, and that there is no trace of segregation 
in the F, generation. 
The above statements have been added here to show the 
possible precautions to be observed while interpreting the results 
given in the present paper. 
- J am however confident that the rats examined here were free 
from all gross abnormalities and furthermore it is my impression 
