164 



ISLAND LIFE. 



[part I. 



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 partially 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, a 

 fresh 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 excentricity 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 excentricity for three million years back ; 

 and from these it appears that there have been many periods 

 of high excentricity, 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 smd perihelion was thirteen and a half millions of miles, 

 whereas during the last glacial period the maximum difference 

 w^as 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 

 part of the latter period ; and Dr. Croll maintains that a glacial 

 epoch must then have occurred surpassing in severity that of 

 which we have such convincing proofs, and consisting like it of 



1 London, Edinhurgh and Dublin Philosophical Magazine^ Vol. XXXVI., 

 pp. 144-150 (1868). 



