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 
