66 HISTORY OF THE EUROPEAN FAUNA. 



J. Geikie one of our highest authorities on this 

 subject a great part of Northern Europe became 

 practically uninhabitable owing to the severity of 

 the climate. 



To enable us to judge better of the true value of 

 the many hypotheses which have been advanced 

 to account for this supposed extraordinary fall of 

 temperature during the "Ice Age," we must compare 

 the views of other authorities with the one just 

 quoted. I do not propose to discuss the causes which 

 have led to the production of the Glacial period 

 those interested in these questions should consult 

 the writings of Dr. Croll, Professor J. Geikie, Pro- 

 fessor Bonney, Mr. Falsan, and others but merely to 

 give the climatic aspects from a physical, zoological, 

 and botanical point of view. 



According to Professor Penck (a, p. 12), the nature 

 of the glacial climate can be determined by comparing 

 the snow-line of the Glacial period with that of the 

 present day. The position of the snow-line is de- 

 pendent on two climatic factors viz., precipitation 

 and temperature. We know the height at which 

 snow must have lain permanently during the Glacial 

 period, or during the maximum phase of glaciation. 

 If the Ice Age had been produced solely by an 

 increase of snowfall, as has been suggested, Professor 

 Penck tells us that then it must have snowed three or 

 four times as much as it does now. But he does not 

 adopt the view that the Ice Age is due to an increase 

 of snowfall alone. His calculations, based upon the 



