BRAINS OF EUROPEAN AND AMERICAN RATS COMPARED 95 
There is still to be noted another dissimilarity between the 
percentage differences of the cranial capacities (table 19) as com- 
pared with the corresponding differences in brain weight (table 20). 
This dissimilarity also is due to the manner in which the cranium 
shrinks—in this instance according to age. It will be seen by 
looking at table 20 that the percentage difference in the brain 
weight increases regularly with increasing body weight. On the 
other hand, the corresponding records for cranial capacity in table 
19 indicate a decrease with increasing body weight. Here again 
we should have expected the cranial capacity records to behave 
in the same way as the brain weight records, but they do not. We 
find the explanation for this disagreement in the shrinkage of the 
crania as influenced by age. The argument is as follows: 
As is well known, in any series of crania those from the younger 
animals contain a larger proportion of water as well as more 
organic matter and have thinner bones and hence shrink rela- 
tively more when dried than do the crania from the older ani- 
mals. We assume then that in any series of crania, loss of water 
and thickening of the bones increases with advancing age, and 
concomitantly, shrinkage on drying decreases with advancing age. 
In the heaviest body weight group of the albinos (table 19) 
the crania are the more mature and so shrink less than the crania 
of the Norways of like body weight—which are somewhat less 
mature. This statement is based on the fact that the Norway 
rat, although it has probably the same span of life as the Albino 
(Donaldson and Hatai, 711) has nevertheless a much greater range 
in body weight and hence in general for a given body weight it 
must be younger than the corresponding Albinos, although the 
difference in the relative shrinkage for the heaviest body weight 
groups may be absolutely small. It follows from what has been 
said about the range of body weight in relation to the span of life 
in the Norways that an equal diminution in mean body weight, 
say 50 grams, implies a greater diminution in age for the Albino 
than for the Norway. According to the foregoing reasoning, this 
should be followed by a relatively increasing shrinkage in the 
albino crania, and this is the interpretation of the values given in 
the last column of table 19. 
