vi PREFACE 



function of circumstance ; the other persistent and general, 

 a function of time. Within itself a fauna is in a constant 

 state of uneasy restlessness, an assemblage of creatures which 

 in its parts ebbs and flows as one local influence or another 

 plays upon it. It may be that a succession of favourable 

 seasons breeds many field-voles, and the tide of the field-vole 

 race flows to its high water-mark of numbers. But this new 

 food-supply brings to the feast hungry owls, hawks, stoats 

 and others, and as the tide of the beasts and birds of prey 

 flows, that of the voles ebbs. Yet no sooner is the ebb 

 apparent than the carnivores themselves decline for lack of 

 food; and eventually the dead level is reached again. So 

 the story goes on there is a constant ebb and flow of parts 

 within the whole, a fauna is in unstable equilibrium, the 

 " balance of nature " is never quite struck. 



But while the parts fluctuate, the fauna as a whole follows 

 a path of its own. As well as internal tides which swing to and 

 fro about an average level, there is a drift which carries the 

 fauna bodily along an irretraceable course. While the former 

 adjustments depend on temporary influences, such as adverse 

 or favourable seasons or variations in the amount of food- 

 stuffs, the latter is a secular phenomenon, due it may be to 

 climatic changes or to the ordinary processes of organic 

 evolution, and leaving a slowly marked but permanent imprint 

 on the sum total of the fauna. The extinct animals and lost 

 faunas of past ages illustrate the reality of the faunal drift. 



Now, part of Man's influence, where it is inconstant in 

 tendency, is of no more import in the long run than the 

 internal tides of the fauna ; but it is strikingly true that the 

 greater part of his influence ranks with the great secular 

 changes. For his interference tends to persist in fixed 

 directions, and so impels individuals in the fauna and the 

 fauna as a whole upon a definite path along which there is 

 no return. So sweeping are the changes wrought by Man 

 and so swift are they in their action that they obscure and 



