LENGTH OF THE DAY. 41 



mentally connected with the muscular and ner- 

 vous powers, whatever the nature of these may 

 be. The necessity of these alternations is one of 

 the measures of the intensity of those vital ener- 

 gies ; and it would seem that we cannot, without 

 assuming the human powers to be altered, suppose 

 the intervals of tranquillity which they require to 

 be much changed. This view agrees with the 

 opinion of some of the most eminent physiologists. 

 Thus Cabanis* notices the periodical and isochro- 

 nous character of the desire of sleep, as well as 

 of other appetites. He states also that sleep is 

 more easy and more salutary, in proportion as 

 we go to rest arid rise every day at the same 

 hours ; and observes that this periodicity seems 

 to have a reference to the motions of the solar 

 system. 



Now how should such a reference be at first 

 established in the constitution of man, animals, 

 and plants, and transmitted from one generation 

 of them to another? If we suppose a wise and 

 benevolent Creator, by whom all the parts of 

 nature were fitted to their uses and to each other, 

 this is what we might expect and can understand. 

 On any other supposition such a fact appears 

 altogether incredible and inconceivable. 



* Rapports du Physique et du Moral de i'Honmie, II. 37J. 



