14 BRITISH BIRDS. 



in colonies but are diffused in the breeding season over 

 the larger part of a continent, the influence governing the 

 selection of a breeding-site is the same as in the case of 

 birds which nest in colonies, namely, the approach of 

 the breeding season. The cause of the seasonal movement 

 is thus beyond question physiologic, and hence periodic, 

 and irresistible. 



If climatic conditions were everywhere nearly uniform 

 there would doubtless be no migration, as there is now 

 practically no true migration among the indigenous birds 

 of sub-tropical and inter- tropical latitudes near sea-level. 

 The difference in physiographic conditions due to 

 differences of altitude and latitude gives rise, of course, 

 to the widely diverse biotic conditions of different parts 

 of the earth's surface. It is generally believed that in 

 pre-glacial times biotic conditions were vastly less 

 diversified than at present ; it hence seems reasonable 

 to infer that the breaking up of this uniformity of 

 climatic conditions by the glacial period greatly changed 

 the distribution of animal and plant life, which to a large 

 extent must have been either driven southward or ex- 

 terminated over the glaciated regions. With the re- 

 cession of the ice the habitable area at the northward 

 became gradually extended, with marked seasonal 

 changes from pre-glacial conditions. There was now 

 established a distinct alternation of summer and winter, 

 so that many birds found congenial homes in summer in 

 districts which were uninhabitable to them in winter. 

 As the recession of glacial conditions was gradual, the belt 

 of new habitable land must have been at first narrow 

 and the later increase gradual. Here, then, were just 

 the conditions to develop a migratory habit in such birds 

 as were able to extend their summer range to the north- 

 ward. The necessity for migration would vary with 

 different species in accordance with the character of their 

 food and their adaptability to new conditions. Birds 

 organized to subsist upon insects and pulpy fruits would 

 find such food in summer far to the northward of where 



