24 
CURT P. RICHTER 
Only as he grows older does he begin to stay awake in the dark. 
Gradually he becomes more and more a night animal until in 
ripe old age much of his activity is manifested during the hours 
of twilight or the night. The progressive tendency toward 
nocturnal habits with age in the rat may be explained in a num¬ 
ber of ways, all of them, however, quite unsatisfactory. First, 
as the animals grow older they find that they can forage and climb 
about more freely, with less molestation at night than during the 
day. This would certainly be true in part anyway for animals 
living in the open, but for animals living in the laboratory all the 
time from birth on, it seems quite questionable. Then from the 
point of view of the recapitulation theory, it might be said that 
these results would show that the Albino belongs to a species 
which is in the progress of changing its habitat from places on 
the surface of the earth, trees and bushes, to burrows and holes 
in the dark under the ground. According to this theory as it is 
generally understood the diurnal activity would represent a 
former stage in the life of the rat, just as it is said that the grasp¬ 
ing reflex of the human infant belongs to an earlier stage in the 
development of man. Such theories do not seem to be very 
well-founded. A further possible explanation of the progressive 
change toward nocturnal activity may be sought in the changes 
of the structure of the eye with age, but about this very little is 
known. 
V. RELATION OF ACTIVITY TO AGE 
It was shown in the previous chapters how spontaneous activity 
of the rat is modified by the intake of food, by temperature and 
by illumination. It was brought out incidentally in a number of 
places in these chapters that the expression of activity is also 
dependent on age. This relation between activity and age will 
now be examined in more detail. In the first place a determina¬ 
tion was made of the actual amount of activity at the different 
stages of the animahs life. This necessarily included also a deter¬ 
mination of the age at which the animal is at its maximum of 
spontaneous activity. 
