40 TERRESTRIAL ADAPTATIONS. 



no one can doubt that the inclination to food and 

 sleep is periodical, or can maintain, with any 

 plausibility, that the period may be lengthened or 

 shortened without limit. We may be tolerably 

 certain that a constantly recurring period of forty- 

 eight hours would be too long for one day of 

 employment and one period of sleep, with our 

 present faculties ; and all, whose bodies and 

 minds are tolerably active, will probably agree 

 that, independently of habit, a perpetual alter- 

 nation of eight hours up and four in bed would 

 employ the human powers less advantageously 

 and agreeably than an alternation of sixteen and 

 eight. A creature which could employ the full 

 energies of his body and mind uninterruptedly 

 for nine months, and then take a single sleep of 

 three months, would not be a man. 



When, therefore, we have subtracted from the 

 daily cycle of the employments of men and 

 animals, that which is to be set down to the ac- 

 count of habits acquired, and that which is occa- 

 sioned by extraneous causes, there still remains a 

 periodical character ; and a period of a certain 

 length, which coincides with, or at any rate easily 

 accommodates itself to, the duration of the earth's 

 revolution. The physiological analysis of this 

 part of our constitution is not necessary for our 

 purpose. The succession of exertion and repose 

 in the muscular system, of excited and dormant 

 sensibility in the nervous, appear to be funda- 



