104 The Australasian Scientific Magazine. [Oct. i, 1885. 
Yet 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 alternation 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 
account of habits acquired, and that which is occasioned 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 ot 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 fundamentally connected with the 
muscular and nervous powers, whatever the nature of these may be. The 
necessity of these alternations is one of the measures of the intensity of 
those vital energies ; and it would seem that we cannot, without assuming 
the human powers to be altered, suppose the intervals of tranquility 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 isochronous 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 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 
