CHAP. xxv. Do Fish Sleep ? 353 



any number of salmon, asleep aye, fast asleep the first obvious question 

 would be, ' How do you know they were asleep ? ' We must first be agreed, 

 then, as to what sleep is, and what are the indications of sleep that may be 

 accepted in settlement of the question. The ordinary sign of being asleep 

 in a mammal is closing the eyelid, but this indication of sleep you cannot 

 expect to see in a salmon, simply because it has no eyelid, and the absence 

 of any eyelid or nictitating membrane would seem to argue further that the 

 salmon had no need for it, and consequently no need for sleep, which is 

 ordinarily accompanied in mammals by a closing of the eyes, so as to shut 

 out the light and free the brain from its exciting influence. Still it does not 

 seem safe to conclude that because a salmon is not formed so as to sleep 

 under the same conditions as a mammal, therefore it cannot sleep at all, 

 and though the primd facie probabilities may be strongly against a salmon 

 sleeping, as we ordinarily think of sleep, yet they are hardly conclusive, and 

 there are other considerations which may, in their turn, make us lean to the 

 opposite view. 



" The prior question seems to be, what is sleep ? and it may help us if 

 we go back to it. Sleep is common to mammals and birds, and, as far as 

 we can judge from its relations to them, it would seem to be a cessation, 

 more or less complete, of the voluntary energies of the body, and especially 

 of the brain, for the purpose of staying waste or allowing of recuperation 

 and to recur periodically. But the duration, the completeness, the periodi- 

 city, the purpose, all vary so widely in different species, individuals, and 

 ages that we have been able to observe, that we are led to. the unavoidable 

 conclusion that in all probability there are still as many varieties of sleep 

 in the regions beyond our observation as there are within it. For instance, 

 among human beings the sleep of an infant is much heavier and longer than 

 that of a healthy mature man, the new-born infant sleeping almost con- 

 tinuously, the man for six or eight hours in the twenty-four. In the infant 

 the duration of sleep required would seem to be in some measure connected 

 with growth. Then consider the widely-differing quantities of sleep taken 

 by the same individual bird in summer and winter. Here the duration of 

 sleep would seem to be connected with the presence or absence of the means 

 of procuring food for the recuperation of the system in place of sleep, and 

 also with the exciting powers of light. But light is by no means a constant 

 factor, many birds and mammals being more active by night than by day. 

 Digestion seems to be another purpose, if we notice how young chickens 

 always nestle under the hen directly they have had a full satisfying meal, 

 and carnivora sleep off a gorge, and some humans find a post-prandial nap 

 irresistible. Heat and cold are again other factors ; witness the sleeps of 

 aestivation and hybernation, observable, the former in the crocodile and the 

 fish that live for months in a torpid state under the sun-dried clay of an 

 eastern clime, the latter in the bear, the hedgehog, the dormouse, etc. How 

 different again are the degrees of suspension of energy. In the sleeps of 

 aestivation and hybernation it approaches very closely to a complete loss of 



THE ROD IN INDIA. 2 A 



