LENGTH or 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, xivithout

assuming thehuman 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 and rise every (lay at the safne hours; and observes that this periodicity seems t0 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 (lu Physique ct du llloral de Yllomme, ll. 371.