98 ANIMAL ECOLOGY 



seen that each place has several fairly distinct communities 

 (distinct in characteristic composition of species, not in the 

 sense of possessing entirely different species) which come out 

 and transact their business of feeding and breeding at different 

 times. We have further seen that the changes in the environ- 

 ment v^hich cause this division into communities at different 

 times are in many cases regular and rhythmical, so that it is 

 possible to classify the latter into definite types — day and night, 

 high and low tide, wet and dry weather, winter and summer, 

 and so on. In spite of the comparatively regular nature of 

 these changing communities, they make the study of this side 

 of ecology excessively complicated, and it is almost impossible 

 to work out the food-cycles, etc., of any ordinary well- developed 

 community of animals with anything remotely approaching 

 completeness. The field worker is faced with masses of pre- 

 liminary routine work in the way of collecting, etc., with little 

 chance of getting on with a more fundamental study of the 

 problems he is continually coming across. The incredibly 

 intricate and complex nature of a fully developed community 

 of animals is a really serious obstacle which has to be faced, 

 especially as most ecologists are unHkely to be able to obtain 

 the help of more than one or two others. 



24. It is therefore desirable, and in fact essential, that any 

 one who intends to make discoveries about the principles 

 governing the arrangement and mode of working of animal 

 communities, should look round first of all with great care, 

 with the idea of finding some very simple association of animals 

 in which the complications of species and time changes are 

 reduced to a minimum. The arctic tundra forms just such 

 a habitat. Here time-changes are practically ruled out {i.e. 

 there is only one community in each habitat, and it forms a 

 comparatively homogeneous unit). Experience has shown 

 that it is quite possible for one person to study with reasonable 

 ease the community-relations of arctic animals, in a way that 

 would be entirely impossible in some more complicated place 

 like a birch wood.^^ In our own latitudes simple communities 

 may also be found in certain rather peculiar habitats such as 

 brackish water and temporary pools ; and it seems certain that 



