BRAIN WEIGHTS OF RATS 565 
that, provided the dietary factor was properly adjusted to the 
animal, the Norway rats may not have suffered any considerable 
physical alteration during the short period of captivity (two 
years or more). 
One peculiar phenomenon appearing in the present study was 
that the range of variability in the brain weights for any given 
body length was noticeably greater in the hybrids than when 
either the Norway or albino rats alone were treated. Whether 
this wide range of brain weight distribution is characteristic 
for the hybrids in general, or whether it was due to the effects of 
captivity (ncluding the dietary factor) on normal growth of 
the brain in weight is difficult to decide. We must await fur- | 
ther observations. 
CONCLUSIONS 
Despite the fact of the several difficulties already mentioned 
we may draw the following conclusions: 
1) The hybrid offspring F; and F, from Norways x Albinos 
possess brain weights which are intermediate between the brain 
weights given by the norway and albino parents respectively. 
2) The frequency distribution of the brain weights in I’; and 
I’, is symmetrical round the mean brain weight, and is closely 
represented by the normal probability curve. There is no evi- 
dence of segregation. 
3) It appears that there is no definite correlation between the 
size of the brain and the character of coat color; that is, the 
pigmented hybrid rats possess brain weights similar to those 
of the non-pigmented hybrid rats. 
