CHAP. X.] THE RATE OF ORGANIC CHANGE. 



223 



is all that is necessary to keep the processes of " natural 

 selection " in constant operation. 



The frequent recurrence of periods of high and of low excen- 

 tricity must certainly have produced changes of climate of 

 considerable importance to the life of animals and plants. 

 During periods of high excentricity with summer in perihelion, 

 that season would be certainly very much hotter, while the 

 winters would be longer and colder than at present ; and al- 

 though geographical conditions might prevent any permanent 

 increase of snow and ice even in the extreme north, yet we 

 cannot doubt that the whole northern hemisphere would then 

 have a very different climate than when the changing phase of 

 precession brought a very cool summer and a very mild winter 

 — a perpetual spring, in fact. Now, such a change of climate 

 would certainly be calculated to bring about a considerable 

 change of species, both by modification and migration, without 

 any such decided change of type either in the vegetation or 

 the animals that we could say from their fossil remains that any 

 change of climate had taken place. Let us suppose, for in- 

 stance, that the climate of England and that of Canada were to 

 be mutually exchanged, and that the change took five or six 

 thousand years to bring about, it cannot be doubted that con- 

 siderable modifications in the fauna and flora of both countries 

 would be the result, although it is impossible to predict 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, while 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 production 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 



