24 THE MIGRATIONS OF BIRDS 



hatch, a species that comes south in abundance at 

 irregular intervals. He considers that these flights 

 take place only when the species has increased to a 

 point where it is crowding its northern range. It 

 then scatters over a broad area and is found in 

 abundance in regions to the southward. He con- 

 siders that the majority of these migrants fail to re- 

 turn to their original home. Such movement he 

 terms centrifugal, in distinction from a centripetal 

 movement in which a species has separate breeding 

 and wintering grounds between which its individuals 

 perform regular migratory flights. He considers the 

 condition of indiscriminate wandering the original 

 one, in which the bird may succeed in returning oc- 

 casionally to its original home, perhaps from a short 

 distance only. With this established as habit, the 

 distance between the winter and summer homes may 

 become greater and greater until the area inhabited 

 in winter may lie without the limits where the 

 species breeds. This in brief is an outline of what has 

 unquestionably taken place in the development of 

 migratory habit, without attempt at explanation of 

 the principles that have controlled it. 



A somewhat different belief is alleged by those 

 who support the theory o f phototropism , in which it 

 is considered that birds naturally turn to the region 

 of greatest light and retreat from one in which light 

 is curtailed. According to this belief, migratory 



