SLEEP 159 



as the elementary form the simplest equivalent of the 

 sleep of man. The quiescence which falls on the earth 

 with the setting of the sun has, however, become the 

 opportunity of two different classes of living things to 

 seize an advantage. Beasts of prey, many of them, sleep 

 during the day, and steal forth at night on velvet foot to 

 pounce on the slumbering animals which are their neces- 

 sary food. Another group of timid animals, moths and 

 small beasts like mice, hedgehogs, and lemurs, find their 

 safety in the dark, and only then venture forth. Even 

 so, the moths are met by special nocturnal enemies, the 

 bats. So that the primitive arrangement is complicated 

 by a wakefulness, exchanging day for night. 



It is natural to apply the word " sleep " to the state 

 of profound repose which other living things appear to 

 enter upon at night, so far as we can judge by changes 

 of activity and attitude although it must be remembered 

 that the sleep of man is what we really indicate by that 

 word, and that it is difficult to trace anything beyond a 

 superficial similarity between man's sleep and the repose 

 or quiescence following upon activity in other living 

 things excepting those which by their structure and 

 the working of their mechanism are obviously com- 

 parable to man, such as beasts, birds, reptiles, and fishes. 

 The " sleep of plants " is the term applied to the closing 

 of the flower, the drooping of the flower-head and of the 

 leaves of many of the common flowering plants, which 

 occurs at sunset or during the later hours of sunlight. 

 But it seems that this is not really comparable to man's 

 sleep. The closing of the flower appears to be a pro- 

 tection of its perfume from useless evaporation during 

 the darkness, and the drooping a device to avoid the 

 settlement of dew and the injurious action of cold. 

 Living things always furnish us with examples of 

 adaptations resisting the general law and as there are 



