CLARK : NOCTURNAL ANIMALS 139 



ZOOLOGY. — Nocturnal animals. Austin H. Clark, National 

 Museum. 



Ill the discussion of zoogeographical problems, and especially 

 in the reconstruction of hypothetical land bridges over which ani- 

 imals are assumed to have migrated from one region into another, 

 comparatively little attention has been paid to analyzing the 

 data upon which sweeping generalizations are based. 



We are tempted to consider as a homogeneous unit all of the 

 animals collectively occurring in any one locality, speaking of 

 this unit as the ''fauna" of this locality, and forgetting that in 

 reality it is a heterogeneous collection of animal forms derived 

 from all of the more adaptable and virile types in all the faunal 

 complexes which from the distant past to the present day have 

 swept over the region. 



Since in reality the terrestrial animals of any given locality 

 collectively form a heterogeneous faunal complex, instead of a 

 homogeneous entity, it becomes essential that we should endeavor 

 to find some criterion by which this complex may be separated 

 into its original constituents, or at least whereby a beginning may 

 be made in this direction. 



On land, abundance of light alternates with a more or less com- 

 plete absence of light, and we therefore find many animal types 

 which are strictly diurnal, like most birds, many which are 

 strictly nocturnal, like the bats, and many which are indiffer- 

 ently one or the other, like most insects. 



Nocturnal animals, properly speaking, are animals which, 

 while capable of performing all their normal functions in the day 

 time, and not dependent upon other nocturnal animals, are ac- 

 tive only at night. 



Thus none of the amphibians come within the category of 

 nocturnal animals, for the amphibians are active whenever the 

 humiditj^ is high enough so that they are in no danger of dermal 

 desiccation, whether at night or during rains; similarly, though 

 active chiefly or entirely at night, none of the terrestrial Crus- 

 tacea or molluscs are properly nocturnal. 



]Many of the herbivorous mammals are most active at night, at 

 which time they often make long journeys for water; this is done 



