GLACIAL EPOCHS AND POLAR CLIMATES, 41 



certain as that that grand revolution of the seasons 

 has produced changes in the past. At the present 

 time the northern temperate zone with winter in peri- 

 helion combined with a low degree of eccentricity is 

 favoured with a comparatively mild climate, and 

 this, in the normal course of things, will be enjoyed 

 for some thousands of years. Then, however, with 

 winter in aphelion, a colder climate will undoubtedly 

 prevail; the winters will become colder, and last 

 longer ; the summers become hotter, and shorter in 

 proportion. These changes would so modify the 

 present climate of the British Islands that, as 

 Wallace suggests, perpetual snow will rest on all 

 our highest mountains. The effect of such an 

 inevitable change of climate will be to banish many 

 of the birds from our islands during winter that are 

 at present resident with us, to cause many other 

 migrants to alter the period of their annual journeys, 

 and to lower the winter range of many boreal forms 

 that at the present time winter in latitudes north of 

 us. This descent of northern forms and banishment 

 of species to southern haunts will also entail many 

 movements among species wintering in them, through 

 changed conditions of life due to competition, and 

 consequently a fiercer struggle for existence. 



It has been suggested by Mr. Seebohm that the 

 migratory habits of the birds comprising the great 

 Polar family Charadriid.^ were initiated by the 

 want of light during the North Polar winter (or the 

 season analogous to such, for no great change of 

 temperature then occurred to separate that period 



