TIME AND ANIMAL COMMUNITIES 87 



time of feeding ; or else they come out chiefly at dusk and so 

 form a transition from one to the other. Also many animals 

 have regular habits which do not correspond exactly with day 

 and night, owing to the fact that the thing controlling them 

 is not light or heat but something else, such as rain or other 

 weather conditions. 



5. Such day and night changes are not found in free-living 

 animals only, but also exist among parasites of mammals, and 

 probably of birds too. Owing to the fact that most mammals 

 sleep either by day or by night there exist corresponding 

 rhythmical changes inside their bodies, especially in tem- 

 perature. Both in birds and mammals the body is slightly 

 colder during sleep than when they are awake. This rhythm 

 depends entirely upon the activity of the animal, since nocturnal 

 birds like owls have the normal rhythm reversed {i.e. they are 

 warmer at night), and this in turn can be reversed by changing 

 the conditions under which they live so as to cause the birds 

 to come out by day and sleep by night. Now there are certain 

 round-worms (nematodes) parasitic in man which show the 

 effects of the sleep rhythm in a very remarkable way. The 

 first species (Filaria bancrofti) lives as an adult in the 

 lymphatic glands of man in tropical countries, but its larvae 

 live in the blood. In the daytime these larvae retire to the 

 inner parts of the body, mostly to the lungs ; but at night they 

 issue forth into the peripheral circulation, appearing first 

 about five to seven in the evening, reaching a maximum about 

 midnight, and disappearing again by about seven or eight in the 

 morning. This rhythm can be reversed if a person stays up 

 all night and sleeps in the day, which shows that the nematode's 

 activity is affected by rhythmical changes in the conditions of 

 the body like those which we have described above. Another 

 species of Filaria (called Loaloa)J[i2iS larvae which live in the 

 blood of man, but unlike the other species these larvae come 

 out only in the day, disappearing at night. It is stated that 

 this periodicity is not affected by reversal of sleep, but pre- 

 sumably it must originally have been caused by some rhythm 

 in the bodily environment.-^^^ A third species has larvae in 

 the blood which occur in the peripheral circulation equally 



