Animals J3efore Man 



changed, tropical animals died out, and the 

 present forms arose to take their places. 



This cooling off was not continuous, for 

 there were great fluctuations of temperature, 

 and between the periods of cold were intervals 

 of warmth. One natural effect of this seems 

 to have been a shifting back and forth of the 

 boundaries between southern and northern ani- 

 mals, so that at one time there were tapirs in 

 Tennessee, while during the greatest cold musk- 

 oxen came as far south as Kentucky. 



It is largely due to this that the lines be- 

 tween the Pliocene and Pleistocene, and be- 

 tween that and the present period, are not more 

 sharply drawn. For these periods are not 

 clearly distinguished from one another either 

 by the character of the formations or by the 

 fossils they contain. 



The formations consist of sands, gravels, 

 marls, and peat bogs, and the animals contained 

 in them are for the most part not unlike those 

 living, the marked differences being brought 

 about by the dying out of some of the larger 

 forms. 



252 



