TIME AND ANIMAL COMMUNITIES 99 



our knowledge of the social arrangements of animals will be 

 most successfully and quickly advanced by elaborate studies 

 of simple communities rather than superficial studies of com- 

 plicated ones. 



In order to avoid misunderstanding it should be pointed 

 out that the latter type of work is of great value and interest in 

 other ways, above all for the light that it throws upon other 

 aspects of ecology, e.g. distribution of species, or ecological 

 succession. The statements made above apply only to eco- 

 logists who intend to study the principles of social relations 

 among animals, and it must not be thought that the value 

 of general biological surveys is in any way depreciated. The 

 latter will always form a most important part of ecological 

 work in the field. 



25. If we followed the subject of time- communities to its 

 logical conclusion (which happily we shall not do, since it 

 would involve a consideration of astronomy and the causes of 

 ice ages, and finally a discussion of the evolution of man), we 

 should have to consider the larger periodic variations in climate 

 from year to year, which undoubtedly exist, even though 

 opinions may differ as to their exact cause and periodicity. 

 For instance, in England there were severe droughts in 1899, 

 191 1, and 1 92 1. In the same way there have been extremely 

 wet years, or very cold winters (see p. 131), all of which have 

 enormous effects upon wild animals. These periodic variations 

 in the climate and weather have chiefly an influence upon the 

 numbers of animals by encouraging or discouraging their 

 increase, and therefore to some extent their distribution. 



26. The exact limits of the ranges of a number of animals 

 are constantly shifting backwards and forwards, ebbing and flow- 

 ing as the outer conditions change, and as the numbers of each 

 species increase or decrease. We. understand at present little 

 about the precise causes of these fluctuations in range ; but 

 although the immediate influence at work may often be biotic, 

 many of these changes are no doubt ultimately referable to 

 short-period cHmatic pulsations, whether irregular or regular. 

 For instance, in certain years there are great influxes into the 

 British Isles of various animals not normally found there, or 



