482 GENERAL CONSIDERATIONS 



is filling up from the bottom. As the shores become firm the bog and 

 marsh plants give way to shrubs and trees, and ultimately a forest may 

 cover the area earlier occupied by the lake. In some cases grasses may 

 become so firmly established that the forest is unable to enter, and then 

 the lake is represented by a grassy area of flat prairie surrounded by 

 forest. With all these changes it is inevitable that there should be a 

 corresponding succession in the types of animal communities (Fig. 301). 

 Corresponding periods may be observed in any permanent body of water. 

 In addition to such succession as has been referred to and which 

 involves long periods of time there are annual, or seasonal changes, which 

 take place at intervals during the year and are more or less dependent 

 upon the season. In the tropics where conditions are very uniform there 

 are no marked seasonal changes. In the polar regions, too, where condi- 

 tions change with relative abruptness from winter to summer and back 

 to winter again, there are no marked seasonal gradations. In the tem- 

 perate zone, however, where the four seasons are of more nearly equal 

 length and the transition from one to another is gradual, definite and 

 orderly seasonal changes take place. 



536. Rhythms. — Somewhat similar to succession are the rhythms 

 that present themselves in animal communities, as a result of which many 

 of the animals belonging to that community never meet. Among these 

 rhythms is the night and day rhythm, as a result of which the animals 

 which are active during the night form a group distinct from those active 

 during the day. This is most strongly marked in deserts, where the 

 conditions between night and day are most different. There is no night 

 fauna in the polar regions because there the night is too cold to permit 

 any activity. Night faunas are, on the other hand, exceedingly rich in 

 the tropics where the heat of the day enforces quiet on animals and the 

 night is the time of maximum activity. Other rhythms are those seen 

 along the seashore and connected with tidal currents and those, in the 

 case of terrestrial animals, involving dry and wet weather. 



It has recently been recognized that the abundance of many animals 

 varies over a period of years in a rhythmic manner, and these rhythms 

 seem to be related to predatory enemies and disease producing organisms. 

 As an animal increases in abundance its enemies also increase; when its 

 numbers reach a maximum these enemies gather in maximum numbers, 

 epidemics of disease develop, and the animal may be so reduced in num- 

 bers as to become scarce. This results in a reduction of the enemy forms, 

 which affords the animal the opportunity again to increase. However, 

 many years must pass before it can regain its former numbers. Then the 

 process is repeated. 



537. Marine Faunas. — When one takes up the subject of faunas he 

 finds himself on a border line between ecology and zoogeography. The 

 modifications and the adjustments which marine animals show to varying 



