102 ANIMAL ECOLOGY 



and the ways in which they are regulated, and show how a 

 great many of the phenomena connected with numbers owe 

 •their origin to the way in which animal communities are 

 arranged and organised, and to various processes going on in 

 the environment of the animals. We shall first of all try to 

 give a picture of the enormous numbers of animals, both of 

 individuals and species, that there are in every habitat ; then 

 we shall describe the great powers of increase which they 

 possess. This leads on to the question of what is the desirable 

 density of numbers for different animals ; and it will be seen 

 that the whole question of the optimum number for a species 

 is affected by the unstable nature of the environment, which is 

 always changing, and furthermore by the fact that practically 

 no animals remain constant in numbers for any length of 

 time. We have further to inquire into the effects of varia- 

 tions in numbers of animals, and into the means by which 

 numbers are regulated in animal communities. The final 

 conclusion to which we shall come is that the study of animal 

 numbers is as yet in an extremely early stage, but that it is one 

 of profound importance both theoretically and practically, 

 and one which can best be studied through the medium of 

 biological surveys and study of animal communities, from the 

 point of view of food-cycles and time- communities. 



2. It is rather difficult to realise what enormous numbers 

 of animals there are everywhere, not only in species but in 

 number of individuals of each species. The majority of 

 animals, especially in this part of the world, are small and 

 inconspicuous — ^it is estimated that at least half the species in 

 the animal kingdom are insects — and their presence is therefore 

 not very obvious. If you ask the ordinary person, or even 

 the average naturalist, how many animals he thinks there are 

 in a wood or a pond or a hedge, his estimate is always sur- 

 prisingly low. Two boys of rather good powers of observation 

 who were sent into a wood in summer to discover as many 

 animals as they could, returned after half an hour and re- 

 ported that they had seen two birds, several spiders, and some 

 flies — that was all. When asked how many species of all kinds 

 of animals they thought there might be in the wood, one 



