100 ANIMAL ECOLOGY 



only rarely. Well-known examples are the crossbill, the arctic 

 skua, the sand-grouse, the clouded yellow butterfly, and various 

 marine fish. These variations in the numbers of animals, or the 

 arrival of entirely strange species, have a definite effect upon the 

 species-composition of animal communities, but they are less 

 important than such smaller rhythms as the seasons of the year. 

 A consideration of this question leads us on naturally to con- 

 sider the numbers of animals, and the means by which these 

 numbers are regulated. This is a very big subject. It is 

 also a very interesting one, and less is known about it than 

 about almost any other biological subject. The study of 

 animal numbers will form in future at least half the subject 

 of ecology, and even in the present state of our knowledge 

 it seems worth while to devote two chapters to it. This we 

 shall accordingly do. 



