126 STUDIES IN LIFE AND SENSE. 



glaciers and snows. Geology has no historical or absolute chronology, 

 and the duration of the ice age may not be set down in years. That 

 it was a period of extensive duration, however, there is no reason to 

 doubt, and when it passed' away it was succeeded in due time by the 

 temperate climate we now enjoy. The effect such alterations of 

 climate would have upon animal life may readily be conceived. 

 Retreat from north to south, as the ice age advanced its chilly 

 snow-sheet, would be the order of the day, and the extent of the 

 southward journeying would be determined conjointly by the rate of 

 advance of the ice-fields and by the failure of food. The renewal 

 of the genial climate would result in the northward journeying of the 

 birds, which, having become accustomed through long ages to a 

 larger area of habitation, would naturally journey to and fro within 

 that area a region of habitation in the case of our own migratory 

 birds extending from the North of Europe to Central Africa, and 

 possibly further south still. Geology, it is true, does not prove much 

 to us from the fossil history of birds, for the remains of birds are few 

 and far between as compared with those of most other animals. But 

 if quadrupeds once denizens of European forests are now extinct 

 therein, and are found represented by living species only in southern 

 and warmer areas, we may readily enough conceive that birds would 

 similarly be driven southwards, and with greater powers of movement 

 and of dispersion by flight would more readily seek and regain their 

 ancient home when the genial climate of to-day succeeded the ice 

 age of the geological yesterday. 



Nor is this all. The instinct which prompts and directs birds to 

 fly from one land to another may be thus regarded as being inaugu- 

 rated by the alternation of cold and warm climates, and as having 

 been inherited and promulgated in some birds, and altered or extin- 

 guished in others. We may, however, learn from geology the plain 

 reason why this instinct had, so to speak, an easier task before it at 

 the beginning of the habit of migration than apparently lies before 

 it now. Before, during, and after the ice age, the boasted inde- 

 pendence of Britain, as far as its isolation from other lands is con- 

 cerned, had no status or existence. Britain was then part of the 

 European continent, and although the broad basin of the Mediterra- 

 nean was probably sketched out, Europe and Africa were one, and 

 were locked together by connecting land. With succeeding years, 

 however, subsidence of land had done its work, and had broken up 

 Europe in the north, and dissociated Ethiopia from Europe in the 

 south. The birds, however, began their migrations over continuous 

 land surfaces, such as exist in the New World of to-day, and the 

 habit and instinct of flight overland thus came to serve the turn of 

 the animal when that land was here and there broken up and when 

 the deep rolled over the sunken world. The instinct acquired in the 



