224 
ISLAND LIFE. 
[rART I. 
excentricity, for it is impossible to doubt that a varying distance 
of the sun in summer from 86 to 99 millions of miles (which 
is what occurred during — as supposed — the Miocene period, 
850.000 years ago) would produce an important difference in 
the summer temperature and in the actinic influence of sun- 
shine on vegetation. For the intensity of the sun’s rays would 
vary as the square of the distance, or nearly as 74 to 98, so 
that the earth would be actually receiving one-fourth less sun- 
heat during summer at one time than at the other. An equally 
high excentricity occurred 2,500,000 years back, and no doubt 
was often reached during still earlier epochs, while a lower but 
still very high excentricity has frequently prevailed, and is 
probably near its average value. Changes of climate, therefore, 
every 10,500 years, of the character above indicated and of 
varying intensity, have been the rule rather than the exception 
in past time ; and these changes must have been variously 
modified by changing geographical conditions so as to produce 
climatic alterations in different directions, and giving to the 
ancient lands either dry or wet seasons, storms or calms, equable 
or excessive temperatures, in a variety of combinations of which 
the earth perhaps affords no example under the present low 
phase of excentricity and consequent slight inequality of sun 
heat. 
Present Condition of the Earth one of exceptional Stability as 
regards Climate. — It will be seen, by a reference to the diagram 
at page 165, that during the last three million years the excen- 
tricity has been less than it is now on eight occasions, for short 
periods only, making up a total of about 280,000 years ; while 
it has been more than it is now for many long periods, of from 
300.000 to 700,000 years each, making a total of 2,720,000 
years, or nearly as 10 to 1. For nearly half the entire period, 
or 1,400,000 years, the excentricity has been nearly double what 
it is now, and this is not far from its mean condition. We have 
no reason for supposing that this long period of three million 
years, for which we have tables, was in any way exceptional 
as regards the degree or variation of excentricity ; but, on the 
contrary, we may pretty safely assume that its variations during 
this time fairly represent its average state of increase and 
