CECO LOGICAL 63 



temperature, rainfall, vegetation, altitude ; while lastly, but by 

 no means leastly, we must reckon with the past — the upheavals 

 and subsidences, and changes of climate which have done so 

 much to change the relative proportions of land and water 

 during the last million years or so. As the history of the 

 rocks bear testimony, land was where now an ocean heaves 

 and swells : what are now continents have more than once 

 been submerged areas. And these upheavals and depressions, 

 as the sequel will show, are responsible for many of the enigmas 

 of distribution to-day. 



There seems good reason to believe that birds, in common 

 with other vertebrates, originated in the northern hemisphere 

 and gradually travelled southwards, forming, as they moved, 

 so many fresh centres of evolution and distribution. This 

 hypothesis explains much that would otherwise appear inex- 

 plicable, and furthermore is supported by the fact that the 

 earliest known fossil birds have been unearthed either in 

 Northern Europe or Northern America. 



With the formation of the existing continents migration 

 became restricted, and new evolutionary centres came into 

 being ; and the number of these increased as the sea, from one 

 cause or another, cut off more or less extensive areas of these 

 continents to form islands, or as oceanic islands came into being 

 and afforded new areas for colonisation. And thus it is that, 

 as a general survey of the larger groups of birds will show, 

 certain more or less distinct ornitho-geographical areas or 

 " regions " may be distinguished. At the present time the 

 regions generally recognised are the : — 



1. Palaearctic. 



2. Ethiopian. 



3. Indian. 



4. Australian. 



5. Nearctic. 



6. Neotropical. 



By some authorities, and with much reason, the Palaearctic and 

 Nearctic Regions are regarded as forming but one great " Hol- 

 arctic" Region. But be this as it may, these several regions 

 are further divided into numerous sub-regions and provinces, 

 the more important of which we shall now proceed to define. 



