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. 1 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 perihelion 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 
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 , Edinburgh and Dublin Philosophical Magazine , Vol. XXXVI., 
pp. 144-150 (18G8). 
