CHAPTER VII 



THE SUCCESSIVE MAMMALIAN FAUNAS OF NORTH AND SOUTH 



AMERICA 



The natural method of telling a story is to begin at the 

 beginning and go on to the end, but to deal in that manner with 

 the many different assemblages of mammals which have in 

 turn inhabited the western hemisphere has the great draw- 

 back of beginning with a time when everything was utterly 

 strange to the modern eye. Could the reader be carried back 

 to the far distant days of the Paleocene epoch, he would find 

 himself in a completely unfamiliar world ; and there is therefore 

 a real practical advantage in reversing the story and starting 

 with the end and thus proceeding gradually from the more 

 to the less familiar. The foregoing chapter gave a sketch 

 of the more striking and characteristic mammals which inhabit 

 the xA.mericas to-day, and we may now take a step backward 

 to the epoch immediately preceding our own, the Pleistocene. 



As was shown in Chapter V, the Pleistocene was a time of 

 many and great climatic vicissitudes, periods of cold, when the 

 northern part of the continent was buried under great ice- 

 sheets, alternating with far milder periods, when the climate 

 was much as at present, or even warmer. These climatic 

 changes necessitated many changes in the distribution of ani- 

 mals and plants, increasing cold driving them southward, while 

 the return of more genial conditions permitted the northward 

 migration of southern forms. The effects of these changes of 

 climate are still plainly visible in the geographical arrangement 

 of living beings in the northern continents and many anomalies 

 of distribution, otherwise inexplicable, are thus made clear. 



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