148 ANIMAL ECOLOGY 



migrations ; but these, again, are usually for the purpose of 

 finding food, etc., and only result secondarily in the spread of 

 the species. Of course this is not to say that some animals 

 have not got very remarkable and specialised means of dis- 

 persal, which exist merely for the purpose of spreading the 

 species. Such adaptations exist in nearly all sessile or very 

 sedentary animals such as marine hydroids on the one hand, 

 or spiders on the other. But the spread of species in an 

 ordinary animal community (excluding coral reef and other 

 marine littoral or intertidal sessile animal communities) usually 

 consists of a rather vague and erratic shifting of the animals 

 from one place to another, which after a number of years may 

 sum itself up as a change in distribution. The cases of large- 

 scale migration in a definite direction are either periodic (as 

 in birds), i.e. they consist in a movement backwards and for- 

 wards between two or more places ; or else are rather excep- 

 tional, as in the case of locust migrations or the outbursts of 

 sand-grouse from Asia. The latter type of migration is very 

 striking both from its size and also from its regular periodicity, 

 but the number of species in which it takes place at all often is 

 probably rather limited. 



3. The idea with which we have to start is, therefore, that 

 animal dispersal is on the whole a rather quiet, humdrum pro- 

 cess, and that it is taking place all the time as a result of the 

 normal life of the animals. A good example of this sort of 

 dispersal is provided by the capercaillie (Tetrao urogallus), 

 which became extinct in Scotland about 1770 but was reintro- 

 duced in 1 837, and has spread gradually, so that it now occupies 

 a very large part of the country again. The manner of its 

 spread has been well described by Ritchie, ^^^ who gives a very 

 interesting map showing the dates of its arrival at various 

 places along the routes of its migration from each centre of 

 introduction. The capercaillie is found almost exclusively 

 in pine woods, and its migration was apparently caused by birds 

 flying occasionally from one wood to the next, and so gradually 

 occupying new territory. Parallel with this process went 

 the increase in numbers, but it is almost certainly wrong to 

 imagine that it was direct " pressure of numbers " that caused 



