30 J. E. LITTLEBOY — THE MIGRATION OP BIRDS. 



authority appears to support the latter alternative. I have before 

 referred to his opinion respecting the causes of northern migration ; 

 it is based on the assumption that the sunny south is the original 

 home of migrants, and the same argument, couched in other words, 

 is repeated at a more advanced stage of his essay. " We must not 

 forget," writes Dr. Weissmann,*' " that there was a time when the 

 animal-life on our hemisphere was altogether different from what 

 it is at present. In the Glacial period central Europe had a colder 



climate than now We shall therefore not be wrong if we 



suppose that very many birds, which now inhabit the central and 

 northern regions of Europe, were at that time wanting, because 

 the climate was too severe for them. They must, therefore, have 

 come subsequently from the south, and with the gradual rising of 

 the temperature there must have been a corresponding steady, but 

 of course very gradual, influx of birds to the north. Just in pro- 

 portion as the ice retreated, would the birds push forward the 

 bounds of their habitat, and in the course of centuries may even 

 have advanced hundreds of miles in this way. Here then we have 

 the first condition of the development of the migratory instinct ; 

 a gradual and steady progress of many species in a northerly 

 direction." 



The task of determining the conditions that prevailed during the 

 Glacial period I leave to our geologists, nor shall I attempt to 

 discuss the uncertain possibilities connected with a pre-glacial 

 avifauna ; but if it be true that the severity of an arctic climate 

 spread, during that epoch, widely over Europe, and subsequently 

 retreated slowly but continuously northward, it is probable that 

 the physical aspect of southern Europe and northern Africa may 

 not, at the period referred to, have differed, to any large extent, 

 from that which now prevails in the northern districts that, on 

 every recurring spring, afford breeding accommodation to millions 

 of migrants. If viewed in this light. Dr. Weissmann's conclusion 

 certainly supports the inference that climate and the consequences 

 which naturally result from it, rather than arbitrary degrees of lati- 

 tude and longitude, must always figure as important items among 

 the causes which determine the natural home of migrants. 



Let us now turn our attention to northern latitudes. Allow me, 

 in the first place, to state an interesting fact, that cannot fail to 

 exercise considerable influence on the decision to be arrived at. 

 It is thus defined by Mr. Secbohm : f — " We may lay it down as a 

 law, to which there is probably no exception, that every bird breeds 

 in the coldest region of its migration." Secondly, the shores of 

 the Arctic Ocean, together with its neighbouring lagoons, abound, 

 during the summer months, with muUusks and small crustaceans, 

 which supply in lavish profusion the requirements of the millions 

 of water- fowl that annually resoi't to them. The abnormal 

 supply of ripe fruit that follows the melting of the snow, a 

 supply which is succeeded with marvellous rapidity by the 



* 'Contemporary Review,' vol. xxxiv, p. 543. 

 t ' Siberia,' p. 244. 



