296 THE GROWTH OF THE BRAIN. 



In the temperate latitudes man is subject to the cycle of 

 the year. This is shown by growth and change in body- 

 weight in the young, and in the mature by alterations in 

 nutrition. But it is in the activating and hyberna- 

 ting animals that there is probably to be seen the full 

 development of those changes dependent on the return- 

 ing seasons changes which are only suggested in 

 ourselves. Down r has observed that in England idiots 

 lose in winter the few acquisitions which they have 

 been able to make during the summer, when there is 

 least tax on their poorly nourished bodies. These 

 seasonal variations, with their long rhythms in nutrition 

 and heat regulation of the body, are again broken into 

 monthly periods. Most marked of these is the menstrual 

 period in the female, with its numerous concomitant 

 changes. 2 In the males, also, there is perhaps a trace 

 of this lunar cycle,3 while under pathological conditions 

 appear a whole series of periodic phenomena, insanities, 

 and the like, impulses to " spree " and " bad days," which 

 also seem to have a common basis in such a monthly 

 rhythm. 



The weekly period is only slightly marked and not 

 organically established. When, however, the daily cycle 

 is examined, many important rhythms become evident, 

 some of which are organically based, but many of them 

 quite dependent on varying social customs. 



With the alternations of daylight and darkness there 

 has developed the corresponding alternation of activity 

 and repose. In normal individuals under the most natural 

 conditions this amounts to an organic habit, and where 

 such a habit has been well established, it is only to be 

 slowly broken. On the other hand, variations in external 



1 Down, Mental Affections of 'Childhood and Youth, 1887. 



2 Ellis, Ma?i and Woman, London, 1894. 



3 Nelson, Am. Journ. of Psychology , 1888. 





