CHAP. X THE RATE OF ORGANIC CHANGE 231 



what the precise changes would be. We can safely say, 

 however, that some species would stand the change better 

 than others, while it is highly probable that some would be 

 actually benefited by it, and that others would be injured. 

 But the benefited would certainly increase, and the 

 injured decrease, in consequence, and thus a series of 

 changes would be initiated that might lead to most 

 important results. Again, we are sure that some species 

 would become modified in adaptation to the change of 

 climate more readily than others, and these modified 

 species would therefore increase at the expense of others 

 not so readily modified ; and hence would arise on the one 

 hand extinction of species, and on the other the pro- 

 duction of new forms. 



But this is the very least amount of change of climate 

 that would certainly occur every 10,500 years when there 

 was a high 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 tem- 

 perature and in the actinic influence of sunshine 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 excen- 

 tricity 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 condi- 

 tions so as to produce climatic alterations in different 

 directions, 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 



