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PART I 



Although the geological evidence is opposed to the belief 

 in early glacial epochs except at very remote and distant 

 intervals, there is nothing which contradicts the occurrence 

 of repeated changes of climate, which, though too small in 

 amount to produce any well-marked physical or organic 

 change, would yet be amply sufficient to keep the organic 

 world in a constant state of movement, and which, by 

 subjecting the whole flora and fauna of a country at 

 comparatively short intervals to decided changes of 

 physical conditions, would supply that stimulus and 

 motive power which, as we have seen, 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 

 excentricity 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 although geographical con- 

 ditions might prevent any permanent increase of snow 

 and ice even in the extreme north, yet we cannot doubt 

 that the wdiole 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 instance, that the climate of 

 England and that of Canada were to be mutually ex- 

 changed, and that the change took five or six thousand 

 years to bring about, it cannot be doubted that consider- 

 able modifications in the fauna and flora of both countries 

 would be the result, although it is impossible to predict 



epochs of life extermination were epochs of cold ; and Dana thinks that 

 two at least such epochs may be recognised, at the close of the Palaaozoic 

 and of the Cretaceous periods — to which we may add the last glacial epoch. 



