TIME AND ANIMAL COMMUNITIES 93 



Sometimes the effect of the tidal rhythm is overridden by 

 that of light and darkness. It appears that many of the corals 

 which form reefs in the tropics only become active and feed 

 at night, closing down during the day.^°'^^ The times at which 

 they can feed at night will sometimes be conditioned by the 

 state of the tides. 



14. The Weather. — Most people are aware nowadays that 

 variations in the weather from day to day are caused by the 

 rather irresponsible movements of centres of high pressure 

 (anticyclones) and centres of low pressure (depressions or 

 storm-centres) in the atmosphere. It is customary to speak 

 as if the controlling factors in weather were the depressions ; 

 but this is only a convention which owes its origin to the 

 natural belief that anything which disturbs our peace and 

 happiness by bringing bad weather is an actively interfering 

 agent, probably the Devil himself. In reahty, the two kinds 

 of pressure areas and their complex relations are equally 

 important, but for convenience we talk in terms of depressions. 

 Changes in weather are associated with depressions travelling 

 over the country, and although the actual path of depressions 

 cannot yet be predicted with certainty, there is a perfectly 

 definite and predictable series of events which accompanies 

 their passage. Generally speaking (in England), a depression 

 produces a zone of rainy weather in its front, and a still larger 

 zone of cloudy weather including and extending beyond the 

 rain-area, while in its rear there is fine weather again. Owing 

 to various complicated factors this ideal series of events is by 

 no means always realised in practice, but the sequence is true 

 on the whole. These changes are accompanied by corre- 

 sponding changes in temperature and humidity of the air, and 

 by variations in the muddiness, hydrogen-ion concentration, 

 etc., of fresh water. 



15. These cycles of weather are of varying length, but are 

 usually of the order of a few days or a week or two, so that they 

 fall in periodicity between tidal or diurnal changes, and the 

 annual cycle of the seasons. They have important effects on 

 animals. Many species are restricted in their activity to certain 

 types of weather. For instance, most mammals avoid rain 



