278 HIGH SCHOOL ZOOLOGY. 



and which belong to Post-Kainozoic Age, show that compara- 

 tively recently, Ontario, with the greater part of North America 

 as far south as Washington, was covered by an Ice-Sheet, so 

 that then the whole country resembled the present desolate and 

 lifeless interior of Greenland. Can it be doubted that such 

 changes taking place all over the world must have had the most 

 important influence on the distribution of life ? 



While we recognize a distinction between tropical, temperate 

 and arctic faunas and floras, and are obliged to infer climatic 

 changes if we find that Magnolias flourished in Spitzbergen dur- 

 ing the Miocene epoch, and that Hippopotamuses wallowed in 

 British rivers in the intervals of successive Ice-Sbeets, yet we 

 must be careful not to attribute too great an influence to climate 

 alone, without taking into account the geographical changes to 

 which the climatic ones are secondary. 



14. That such changes have been both recent and profound 

 will now appear from some of the facts of distribution already 

 cited, and the explanation of these offered by pala3ontology. 

 First, however, let us look into the causes and effects of that 

 Glacial Period, during and after which the surface deposits of 

 Ontario were formed. 



There are certain regularly recurring astronomical conditions, 

 which affect the relative length of winter and summer, and 

 which are dependent (1) on the relative proximity of the poles 

 to the sun during these seasons, and, (2) on the amount of 

 eccentricity of the earth's orbit. The relative proximity to the 

 sun in winter of the North and South Hemispheres is I'eversed 

 every 10,500 years, but the amount of eccentricity varies much 

 more slowly. Now, it is at its minimum ; between 100 and 

 200,000 years ago it was very great, with the result of alter- 

 nating periods of 10,000 years of long, cold winters, and short, 

 hot summers, with the reverse. It is believed that these con- 

 ditions, occurring simultaneously with large elevations of land 



