124 
ISLAND LIFE. 
[rART I. 
eight and a half millions of miles between our distance from the 
sun in aphelion and perehelion (as the most distant and nearest 
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 
{(PROBABLE DURATION OF THE GLACIAL EPOCH^ 
THOUSAND YEARS AGO FROM 
A. D. 1800. 
DIAGRAM OF 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 excentricity is higher. 
The horizontal dotted line marks the present excentricity. The figures show the maxima. 
and minima of excentricity during the last 300,000 years from Dr. Croll’s Tables. 
about eighty thousand years ago) the excentricity was very 
great, reaching a maximum of three and a half times its 
present amount at almost the remotest part of this period, at 
which time the length of summer in one hemisphere and of 
winter in the other would be nearly twenty-eight days in excess. 
Now, during all this time, our position would change, as above 
described (and as indicated on the diagram), every ten thousand 
five hundred years ; so that we should have alternate periods 
of very long and cold winters with short hot summers, and short 
mild winters with long cool summers. In order to understand 
the important effects which this would produce we must ascertain 
