DISPERSAL 147 



it will only be possible here to give a general outline of some 

 of the ways in which it affects animal communities. If we 

 studied the factors which cause the onset of migration in animals 

 like birds or lemmings, we should soon find it necessary to go 

 quite deeply into psychology, in order to find out why animals 

 come together into herds at all, why they sometimes become, 

 as it w^ere, hypnotised or obsessed with the migration impulse, 

 and why they migrate in one direction rather than another. 

 If we studied at all closely the reactions of insects, which lead 

 them to seek a particular food-plant, or to lay their eggs in a 

 particular place, we should soon become involved in remote 

 branches of organic chemistry. Dispersal is especially attrac- 

 tive as a subject for study if only because it is so easy 

 at any moment to escape from the purely biological side of the 

 work, and seek a change in psychology, chemistry, meteorology, 

 or oceanography. At the same time, we are chiefly concerned 

 in this chapter with providing a general orientation towards 

 the problem of dispersal, in so far as it forms a factor of the 

 life of animals. It is a subject which requires clear thinking, 

 since in the usual discussions about the present, and even 

 more the past, distribution of animals, one notices a remarkable 

 lack of comprehension of the extremely complicated processes 

 involved in their spread, whether as individuals or species. 



2. Since most plants cannot move about of their own 

 accord, they usually have some special adaptation for the 

 spreading of seeds or other reproductive products ; whereas 

 animals, possessing the power of active movement on a large 

 or small scale, do not have such highly developed special 

 means of dispersal. Furthermore, their powers of movement 

 are not usually employed for the direct purpose of spreading 

 the species over the widest possible area. Although an animal 

 like a rabbit, or an earthworm, or an earwig, has considerable 

 powers of dispersal, these are directed towards its immediate 

 needs of obtaining food, finding water, or a mate, or avoiding 

 enemies, rather than towards the occupation of new territory 

 for its own sake. The latter often takes place automatically 

 as a result of the former type of activity, and this casual un- 

 directed movement may become elaborated into regular 



