26 



THE GLACIAL EPOCH. 



droll's 

 theory of 

 the Glacial 

 Epoch. 



Arctic cli- 

 mate. 



increase and extend their area of distribution until, instead of one circumpolar species 

 inhabiting the shores of the Polar Sea, a dozen circumpolar species should do so. 



Fourth, a second glacial period to disperse each of these 17 species, and isolate parties 

 of each of thern in various southern regions of various climates, as parties of their common 

 ancestor were isolated, so that the 17 species may become a couple of hundred. 



Fifth, a mild climate at the North Pole, again to allow about 60 of these species once 

 more to return to the family home, and to become in a few instances circumpolar again. 



Sixth, and last, a partial return of the glacial condition of the Polar Sea, so that free 

 intercourse across it should be discontinued, and interbreeding confined to the birds living 

 near Bearing Straits, so that Canadian species should in some cases be only subspecifically 

 distinct from their Siberian allies. 



If Geologists can supply us with these six conditions, we ought to be grateful to 

 them for such valuable aid in the explanation of such an interesting problem as that of 

 the geographical distribution of the Charadriidse, whilst at the same time we ask them to 

 acknowledge the value even of such feeble support as is implied in the harmony between 

 their theories and our hypothesis. 



To ascertain whether this harmony exists, it is necessary to examine briefly the latest 

 theory of the Glacial Epoch. 



Croll's theory appears to rue to account for the occasional occurrence of a glacial 

 epoch very satisfactorily. 



The inclination of the axis of the earth to the plane of the ecliptic causes the annual 

 alternation of continuous day during summer and continuous night during winter at the 

 Poles. At the South Pole, which is at the present time in the middle of a mild glacial 

 period, the heat of summer is insufficient to melt the snow on the land which has accumu- 

 lated during winter ; but at the North Pole the heat of summer melts all the snow which 

 has accumulated during winter on the land near the sea-level, and some portion of that on 

 the Greenland mountains, besides a considerable thickness of the ice on the sea near the 

 Pole. So far as I have been able to learn from my three visits to the Arctic Regions, and 

 from information obtained from the narrations of Arctic travellers, the condition of affairs 

 at and near the North Pole is as follows : — During summer and winter there is very little 

 wind. Nearly all the snow which falls during the year falls in late autumn, and nearly all 

 the rain which falls descends in late spring. The normal condition of the weather in 

 winter is perpetual starlight, and that of summer continuous sunshine. In May and 

 October gales are common, probably having the same origin as the equinoctial gales of our 

 latitudes. At the North Pole every wind is a south wind, and any disturbance of the 

 atmosphere of importance must bring air laden with moisture from the tropics, in October 

 to fall in snow, in May to fall in rain to melt the snow. The question is, How can a glacial 

 epoch be produced ? I think we may at once dismiss the idea that the length or severity 

 of the winter is an important factor. No amount of cold in winter can produce a glacial 

 epoch, unless there is a great mass of snow or ice in which the cold may be stored ; nor, on 



