THE HABIT OF MIGRATION 191 



The discoveries of Darwin and Wallace have placed 

 the facts of migration in an entirely new light, and added 

 a new interest to a subject which has always been one of 

 the most fascinating departments of ornithology. The 

 origin of the habit of migration is still involved in much 

 mystery. It is probably a fact in the history of birds of 

 comparatively modern date. It is not confined to any 

 one geographical region, nor to any one family of birds, 

 nor can we assume that it will be present or absent 

 in every species of the same genus. The birds of the 

 Nearctic region are as migratory as those of the Palaearctic. 

 Many birds visit South America and Australia only during 

 the breeding-season. If we include as birds of the tropical 

 regions those species which visit them after having bred 

 in the cooler regions, they will also contain a considerable 

 proportion of migrants, even though no bird migrates 

 there to breed. We may lay it down as a law, to which 

 there is probably no exception, that every bird breeds in 

 the coldest regions of its migrations. No bird migrates 

 to the tropics to breed because there is no hotter region 

 for it to migrate from. The stories of birds breeding a 

 second time in the place of their winter migration 

 probably have the same scientific value as the stories 

 of swallows having been found hybernating in caves and 

 hollow trees, or of toads having been found in the recesses 

 of otherwise solid rocks. 



Many birds, such as the robin, the blackbird, and 

 others, which are for the most part resident in England, 

 are migratory in Germany. There is every probability 

 that it is only within comparatively recent times that 

 these birds have ceased to migrate in England, and we 

 may fairly conjecture that, should the English climate 

 remain long enough favpurable to the winter residence 

 of these birds, they will develop into local races, which 



