CLARK: NOCTURNAL ANIMALS 141 



The strictly nocturnal mammals are the elephants, the hippo- 

 potamus, the tapirs, the rhinoceros, the sloths, Galeopterus 

 (Cynocephalus), the anteaters, the aard-varks, the armadillos, the 

 pangolins, the bats, the opossums, the solenodons, the lemurs 

 (except in Madagascar), and the monotremes (except the 

 aquatic species); among the birds the kiwis and the owl-parrots 

 (Stringops) are nocturnal, and among the reptiles the geckos; 

 among the invertebrates the most striking and isolated nocturnal 

 types are the onychophores and the millipeds. 



A glance at this list of strictly nocturnal types brings out many 

 points of interest. 



Among the birds and mammals the average size is much greater 

 than that of the diurnal types. 



oVIost of the nocturnal mammals have long palaeontological 

 records, and, collectively, the included groups are considered as 

 much more ancient, primitive and aberrant than the diurnal 

 types. 



Taken as a whole the nocturnal animals of the globe indicate 

 zoogeographic affinities very different from those indicated by 

 the diurnal animals; for instance, the nocturnal fauna of America 

 shows a close affinity to that of Africa, and also an affinity with 

 that of Australia, while the occurrence of tapirs, otherwise con- 

 fined to the Malayan region, is of interest; but the affinity indi- 

 cated by overwhelmingly the greater part of the diurnal fauna 

 of America is with eastern Asia. 



The two fundamental differences in the conditions under which 

 the nocturnal animals live as contrasted with the diurnal are (1) 

 the much lower and more uniform temperature, and (2) the 

 more or less complete absence of light. 



It is precisely these two differences which delimit the condi- 

 tions under which the abyssal marine animals live as contrasted 

 with the httoral marine animals, and we are therefore not sur- 

 prised to find that the nocturnal fauna of the land corresponds 

 in its relation to the diurnal fauna exactl}^ to the abyssal fauna 

 of the oceans in its relation to the littoral fauna. 



This comparison enables us to understand how a given type 

 (as for example the bats), exclusively nocturnal in the tropics. 



