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330 THE ZOOLOGIST. 
so by interdependence of habit, from lowest to highest of the 
animal kingdom, the round of night labours and watchfulness 
goes on. : 
In a mere sketch like the present, the subject obviously 
receives but scant justice ; for many interesting nocturnal animals, 
peculiar habits, and curious structural conditions suggest them- 
selves, of which no mention has been made. Nevertheless 
one may be justified in a retrospective summary of the points 
influencing, doubtless to some extent, nocturnal habits. 
All animals suffer waste, and necessarily require food and 
drink to repair waste of tissue. To supply these they must needs 
obtain them at such times and places as they are procurable. 
Animals, moreover, are influenced by the surrounding medium 
and environment generally, and light especially, as well as 
temperature, often cause uncomfortable sensations. 
Many hence endeavour to shun excess of light, and, in so 
doing, seek night or cool twilight to perform their active functions. 
By dependence upon one another for sustenance, by pre- 
cautions of safety against enemies, and occasionally by seasonal 
occurrence of procreative faculties, or the rearing of offspring, 
and by other economic reasons, nocturnal habit may be acquired, 
retained, and ultimately transmitted. 
The organs of sense, more particularly touch, hearing, and 
sight, usually become highly irritable, and, from lowest to highest 
animal forms, sensory apparatus gets specialized. By slight 
alteration in textural qualities functional intensity is superinduced, 
and touch almost amounts to a sixth sense. 
In a number of cases the same group has diurnal and nocturnal 
representatives; and occasionally, so far as research has yet 
shown, no good reason can be given for nocturnal habit. 
Lastly, communities of animals, like human beings, are 
doubtless influenced in a variety of ways; and the inherent 
tendency to aberration, but absolutely from physical causes 
not demonstrable, may act and react in a manner with which we 
are yet unacquainted, 
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