128 



ISLAND LIFE 



PART 1 



factor in the change of magnitude of the excentricity itself. 

 This varies very largely, though very slowly, and it is now 

 nearly at a minimum. It also varies very irregularly ; but 

 its amount has been calculated for several million years 

 back. Fifty thousand years ago it was rather less than it 

 is now, but it then increased, and when we come to a hun- 

 dred thousand years ago there is a difference of eight and 

 a half millions of miles between our distance from the sun 

 in a;phelion and perihelion (as the most distant and nearest 



PROBABLE DURATION OF THE GLACIAL EPOCH 



THOUSAND YEARS AGO FROM 

 A. D. 1800. 



DIAGRAM OP EXCENTRICITY AND PRECESSION. 



The dark and light bands mark the phases of precession, the dark showing short mild 

 winters, and the light long cold winters, the contrast being greater as the excen- 

 tricity is higher. The horizontal dotted line shows the amount of the present 

 excentricity. The figures show the maxima and minima of excentricity during the 

 last 300,000 years from Dr. Croll's Tables. 



points of the earth's orbit are termed). At a hundred and 

 fifty thousand years back it had decreased somewhat — to 

 six millions of miles ; but then it increased again, till at 

 two hundred thousand years ago it was ten and a quarter, 

 and at two hundred and ten thousand years ten and a half 

 millions of miles. By reference to the accompanying 

 diagram, which includes the last great period of excentricity, 

 we find, that for the immense period of a hundred and 

 sixty thousand years (commencing about eighty thousand 



