TIME AND ANIMAL COMMUNITIES 91 



nearly all typical daylight ones. And these form a permanently 

 working community which lives in continuous daylight during 

 the summer, and may in some cases have very little rest for 

 three months — at any rate, as a population. Conversely, below 

 a certain depth in the sea, or in big lakes, and in subterranean 

 waters, and inside the bodies of animals, there is continuous 

 darkness, so that the animals living there also form homo- 

 geneous and permanent communities. Sometimes, however, 

 the bodies of animals reflect the rhythm of their outer environ- 

 ment and cause corresponding differences in their parasite 

 fauna, as in the case of the Filarias already described. 

 Probably the most conservative, smooth- working, and per- 

 fectly adjusted communities are those living at a depth of 

 several miles in the sea ; for here there can be no rhythms in 

 the outer environment, such as there are on land. 



12. As we pass from the poles to the equator the night 

 fauna begins to appear and becomes gradually more elaborate 

 and important, until in such surroundings as are found in a 

 tropical forest it may be more rich and exciting and noisy than 

 the daylight fauna. Alexander von Humboldt ^°^ gives a 

 good idea of this. Camping in the Amazon forests in the 

 early nineteenth century, he wrote : " Deep stillness prevailed, 

 only broken at intervals by the blowing of the fresh-water 

 dolphins. . . . After eleven o'clock such a noise began in the 

 contiguous forest, that for the remainder of the night all sleep 

 was impossible. The wild cries of animals rung through the 

 woods. Among the many voices which resounded together, the 

 Indians could only recognise those which, after short pauses, 

 were heard singly. There was the monotonous, plaintive cry 

 of the Aluates (howling monkeys), the whining, flute-like notes 

 of the small sapajous, the grunting murmur of the striped 

 nocturnal ape {Nyctipithecus trivirgatus, which I was the first 

 to describe), the fitful roar of the great tiger, the Cuguar or 

 maneless American lion, the peccary, the sloth, and a host 

 of parrots, parraquas (Ortalides), and other pheasant-like 

 birds." 



In temperate countries the night-Hfe of animals is by no 

 means so abundant or complex. This may be partly due to 



