ANIMAL INTER-RELATIONS 25 



night. In an oak wood, for instance, there are cer- 

 tain beetles, leaf hoppers, small warblers, and the 

 sparrow-hawk that come out by day. At night there 

 are moths, wood-mice, nightjars, owls, and bats. 

 These diurnal and nocturnal communities are to a 

 large extent independent, although there is overlap- 

 ping of three kinds : some animals are active mainly 

 at dusk — as the noctule bat and one of its foods the 

 dor-beetle, some animals come out both by night 

 and by da}" — as the grass-mouse or vole, while some 

 night animals prey on day animals and while they 

 are at rest, and vice versa — as the barn owl when 

 it raids sparrow roosts at night and the song thrush 

 when it attacks earthworms by day. In spite of 

 these exceptions the broad distinction remains. Defi- 

 nite nocturnal communities are found in all terrestrial 

 habitats except in polar regions and possibly the soil. 

 In fresh water the division is also of importance. In 

 the sea, tidal rhythms play a dominant part in the 

 lives of shore animals, but it seems probable that 

 these forms also have important day and night 

 rhythms which, interacting with the tidal rhythms, 

 may be responsible for some of the curious monthly 

 or fortnightly periodicities in reproduction that have 

 been recorded in a good many marine species (see 

 Munro Fox, 1932, and Amurthalingam, 1928). In the 

 depths of the ocean or in deep lakes, animals Uve 

 permanently in the dark, while Arctic animals hve 

 in permanent dayhght during the summer months. 

 The whole problem of these short rhythms of activity 

 in animals has as yet received comparatively little 

 systematic attention. A careful study of certain 

 nocturnal insects and other invertebrates in a beech- 

 maple forest in Ohio by Park and others (1931) 

 has stressed the importance of internal activity 

 rhythms in insects that come out at night, while 

 similar studies have been made on deer-mice in 

 America (Johnson, 1926) and wood-mice in Eng- 

 land (Elton, and others, 1931) and on fish and other 



