CONCLUSION 189 



biggest part of present-day economic problems in the field, 

 and one of the aims of this book has been to indicate the lines 

 along which the numbers of animals may be studied. The 

 order of the chapters in the present book represents roughly 

 the order in which it is necessary to study the ecology of animals. 

 First, there must be a preliminary survey to find out the general 

 distribution and composition of animal communities. Then 

 attention is usually concentrated on some particular species, 

 with the object of discovering what factors Hmit it in its range 

 and numbers. As far as physical and chemical factors are 

 concerned this can usually be done without reference to other 

 species of animals, but one is practically always brought face 

 to face also with biotic factors in the form of plants and 

 animals. Ecological succession in plants, involving gradual 

 migration of animal communities, has to be studied in this 

 connection. When one starts to trace out the dependence of 

 one animal upon another, one soon realises that it is necessary 

 to study the whole community living in one habitat, since the 

 interrelations of animals ramify so far. The study of an 

 animal community is difficult, since so little work has been 

 done on it ; but there are certain principles which seem to 

 apply to nearly all communities, and which enable the inter- 

 relations of different species to be understood fairly clearly. 

 It is only when the limiting factors, biotic and otherwise, have 

 been appreciated that it is possible even to begin to study the 

 numbers of animals and the ways in which they are regulated. 

 One of the main facts which emerges from this study is that 

 numbers do not usually remain constant for any length of 

 time, but usually vary cyclically, sometimes with extraordinary 

 regularity. Bound up with various aspects of animal com- 

 munities is the question of dispersal of animals, which can be 

 shown to be partly connected .with the ordinary activities 

 of animals, and partly with changes in numbers. Finally, 

 what little we know about the regulation of numbers in 

 animals, enables us to say that the problem of the origin of 

 species can only be successfully solved by the aid of work on 

 numbers. 



The order of study of animals in nature therefore falls 



