140 CLARK: NOCTURNAL ANIMALS 



to avoid overheating and loss of too much water from the body, 

 and, in many places, to avoid certain insects, such as the species 

 of Simulium, Glossina, the Tabanidae, etc., which, larger and 

 more dangerous than any nocturnal insects, always breed in or 

 near water and bite only in the daytime. 



The predacious mammals and birds are active chiefly (though 

 not by any means entirely) at night for the reason that, following 

 the path of least resistance, they always conform to the habit 

 of taking their prey when it is least able to defend itself, and 

 therefore they must be considered as fortuitously, not as truly, 

 nocturnal. 



The day is physiologically the most favorable time for the per- 

 formance of the normal functions of animals, and at the same time 

 it is the period of greatest meteorological and thermal diversity. 

 Therefore it would seem that new types of animals would always 

 first arise as diurnal species. 



If a new animal type arose as a diurnal form, and proved 

 virile and adaptable, it would soon populate all available situa- 

 tions, and would increase so that there would be no room for 

 further additions to its numbers. 



But a type sufficiently virile and adaptable to attain such a 

 condition would most assuredly give rise to crepuscular, and 

 finally to nocturnal, forms. It is at this developmental stage 

 that we find, for example, the rodents and the lepidoptera of 

 the present day. 



A new type of subsequent origin, of the same habits but eco- 

 nomically more efficient, would follow the same course, and there- 

 fore would extirpate the diurnal species of the preceding less 

 efficient type long before it had any effect upon the nocturnal 

 species, so that, when the second type had become fully estab- 

 lished, we would find it consisting of diurnal and crepuscular 

 species, while the older type would consist of strictly nocturnal 

 species, hiding by day and encroaching upon their more eflScient 

 rivals' food suppl}^ at night. 



This is exactly the process by which the deep sea fauna has 

 been formed from the littoral fauna. 



