232 ISLAND LIFE 



excentricity and consequent slight inequality of sun- 

 heat. 



rresent Condition of the Earth One of Exceptional Stability 

 as Regards Climate. — It will be seen, by a reference to the 

 diagram at page 171, that during the last three million 

 years the excentricity 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 de- 

 gree 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 

 decrease during all known geological time. But when the 

 glacial epoch ended, 72,000 years ago, the excentricity was 

 about double its present amount ; it then rapidly decreased 

 till, at 60,000 years back, it was very little greater than it 

 is now, and since then it has been uniformly small. It 

 follows that, for about 60,000 years before our time, 

 the mutations of climate every 10,500 years have been 

 comparatively unimportant, and that the temperate zones 

 have enjoyed an exceptional stahility of climate. During 

 this time those powerful causes of organic change which 

 depend on considerable changes of climate and the con- 

 sequent modifications, migrations, and extinctions of 

 species, will not have been at work ; the slight changes 

 that did occur would probably be so slow and so little 

 marked that the various species would be able to adapt 

 themselves to them without much disturbance; and 

 the result would be an epoch of exceptional stability of 

 species. 



But it is from this very period of exceptional stability^ 

 that we obtain our only scale for measuring the rate of 

 organic change. It includes not only the historical period, 



