142 CLARK: NOCTURNAL ANIMALS 



may become more or less diurnal in the cold northern regions, 

 just as an organism confined exclusively to the abysses in the 

 tropics may become sublittoral, or even littoral, in the antarctic 

 or in the arctic; for the coeflficient of virility and adaptability 

 necessary to enable an animal type to thrive under nocturnal 

 conditions in the tropics, or in the deep sea, is equivalent to 

 that necessary to enable it to exist in unnaturally cold surround- 

 ings, or in unnaturally great diurnal temperature variations. 



In this short paper no attention has been paid to the innumer- 

 able intergrades between the intertidal fauna, and the faunas of 

 the land and of fresh water, or to the singularly instructive faunas 

 of deserts, wet belts, saline lakes, caves, or of abnormal situa- 

 tions in general, nor to isolated island faunas, nor to the charac- 

 ters presented by burrowing animals collectively considered ; nor 

 has attention been given to the singular fact that, parallel to the 

 abstract similarity between the nocturnal terrestrial fauna and 

 the abyssal fauna of the oceans, the fresh water fauna is actually 

 more closely allied to that of the deep sea than either is to the 

 littoral fauna from which both have been derived, but in which 

 the ancestral types have been supplanted by more efficient types 

 of subsequent origin which as yet have not intruded either into 

 the fresh water or into the deep sea; though these points, and 

 many others, have a very intimate bearing upon the problem 

 of the nocturnal terrestrial fauna. 



