170 ISLAKD LIFE 



thousand years that each specially mild period may have 

 lasted, some portions of the north temperate zone, which 

 had 'been buried in snow or ice, would become again 

 clothed with vegetation and stocked with animal life, both 

 of which, as the cold again came on, would be driven 

 southward, or perhaps jDartially exterminated. Forms 

 usually separated would thus be crowded together, and a 

 struggle for existence would follow, which must have led 

 to the modification or the extinction of many species. 

 "When the survivors in the struggle had reached a state of 

 equilibrium, afresh field would be opened to them by the 

 later ameliorations of climate ; the more successful of the 

 survivors would spread and multiply ; and after this had 

 gone on for thousands of generations, another change of 

 climate, another southward migration, another struggle of 

 northern and southern forms would take place. 



But if the last glacial epoch has coincided with, and has 

 been to a considerable extent caused by, a high excen- 

 tricity of the earth's orbit, we are naturally led to expect 

 that earlier glacial epochs would have occurred whenever 

 the excentricity was unusually large. Dr. Croll has 

 published tables showing the varying amounts of excen- 

 tricity for three million years back ; and from these it 

 appears that there have been many periods of high excen- 

 tricity, which has often been far greater than at the time 

 of the last glacial epoch.^ The accompanying diagram has 

 been drawn from these tables, and it will be seen that the 

 highest excentricity occurred 850,000 years ago, at which 

 time the difference between the sun's distance at aphelion 

 and 2yc'^'ihelio7i was thirteen and a half millions of miles, 

 whereas during the last glacial period the maximum 

 difference was ten and a half million miles. 



Now, judging by the amount of organic and physical 

 change that occurred during and since the glacial epoch, 

 and that which has occurred since the Miocene period, it 

 is considered probable that this maximum of excentricity 

 coincided with some j^art of the latter period ; and Dr. 

 Croll maintains that a glacial epoch must then have 



^ London, Edinburgh and Duhlin Philosophical Magazine, \o\. XXXVI., 

 pp. 144-150(1868). 



