152 THE STORY OF THE BIRDS 



have checked avine colonisation in certain direc- 

 tions. It must always be borne in mind that 

 normal dispersal or extension of area is a result 

 of numerical increase ; as the individuals of any 

 species multiply they seek out new quarters and 

 occupy new districts. But the country that this 

 surplus population invades must be suited to 

 the requirements of the species that is gradually 

 invading it. If a supply of food cannot be 

 obtained, especially during the breeding season, 

 if suitable nesting-places are absent, then the 

 colonising movement cannot proceed in that 

 direction. The special requirements of each 

 species determine the general line of its emi- 

 gration. Forest birds could not, and would not, 

 attempt to increase their area across treeless 

 plains ; aquatic birds would only seek to do so 

 in directions (always, however, conforming to 

 the suggested law of dispersal already described) 

 where suitable haunts presented; whilst birds 

 that dwelt upon steppes and open country 

 would follow such country only and colonise 

 it. Then, again, mountain species would keep 

 closely to the mountains in entering new areas 

 of dispersal ; coast birds could only follow the 

 trend of the shore. Then in countless instances 

 as a sedentary species has gradually spread from 



