224 



ISLAND LIFE. 



[part 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. 



Fresent 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 



