THE NORTH POLAR BASIN. 387 



they are now, and even in the British Islands glaciers existed during 

 what has been called the ice age, and the evidence of their existence 

 in the form of rocks, upon which they have left their scratches, and 

 heaps of stones which they have deposited in their retreat, are so obvious 

 that he who runs may read. Similar evidence of an ice age is found in 

 ISTorth America, and to a limited extent in the Himalayas, but in the 

 alluvial i>lains of Siberia and North Alaska, as might be expected, no 

 trace of an ice age can be found. 



Croll's hypothesis that an ice age is produced when the eccentricity 

 of the earth's orbit is unusually great, has been generally accepted as 

 the most plausible explanation of the facts. It is assumed that during 

 the months of summer perihelion evaporation is extreme, and that 

 during the months of winter aphelion the snow-fall is considerably 

 increased. The etfect of the last period of high eccentricity is supposed 

 to have been much increased by geographical changes. The elevation 

 of the shallow sea which connects Iceland with Greenland on the one 

 hand, and the south of Norway and the British Islands on the other, 

 would greatly inc-rease the accumulation of snow and ice in those parts 

 of the Polar Basin where evidence of a recent ice age is now to be 

 found; whilst the depression of the lowlands on either side of the Ural 

 Mountains so as to admit the waters of the Mediterranean through the 

 Black and Caspian Seas, might prevent any glaciation in those parts 

 of the Polar Basin where no evidence of such a condition is now dis- 

 coverable. But this is a (juestion that must be left to the geologist to 

 decide. 



The extreme views of the early advocates of the theory of an ice age 

 have been to a large extent abandoned. No one now believes in the 

 former existence of a Polar ice cap, and possibly, when the irresistible 

 force of ice-dammed rivers has been fully realized, the estimated area 

 of glaciation may be considerably reduced. The so-called great ice age 

 may have been a great snow age, with local centers of glaciation on 

 the higher grounds. 



The zoological evidence as to the nature, extent, and duration of the 

 ice age has never been carefully collected. The attention of zoologists 

 has unfortunately been too exclusively devoted to the almost hoi)eless 

 task of theorizing upon the causes of evolution, instead of patiently 

 cataloguing its effects. 



There is a mass of evidence bearing directly upon the recent changes 

 in the climate of the Polar Basin to be found in the study of the 

 present geographical distribution of birds. The absence of certain 

 common British forest birds (some of them of circumpolar range sub- 

 generically, if not specifically) from Ireland and the north of Scotland 

 is strong confirmation of the theory that the latter countries were not 

 very long ago outside the limit of forest growth. 



The presence of species belonging to Arctic and sub-Arctic general 

 on many of the South Paciiic islands is strong evidence that they were 



