358 HISTORICAL PALEONTOLOGY. 



CHAPTER XXII. 



THE POST-PLIOCENE PERIOD. Continued. 



As regards the life of the Post-Pliocene period, we have, in 

 the first place, to notice the effect produced throughout the 

 northern hemisphere by the gradual supervention of the Glacial 

 period. Previous to this the climate must have been temper- 

 ate or warm-temperate; but as the cold gradually came on, 

 two results were produced as regards living beings of the 

 area thus affected. In the first place, all those Mammals 

 which, like the Mammoth, the Woolly Rhinoceros, the Lion, 

 the Hyaena, and the Hippopotamus, require, at any rate, moder- 

 ately warm conditions, would be forced to migrate southwards 

 to regions not affected by the new state of things. In the 

 second place, Mammals previously inhabiting higher latitudes, 

 such as the Reindeer, the Musk-ox, and the Lemming, would 

 be enabled by the increasing cold to migrate southwards, and 

 to invade provinces previously occupied by the Elephant and 

 the Rhinoceros. A precisely similar, but more slowly-executed 

 process, must have taken place in the sea, the northern Mol- 

 lusca moving southwards as the arctic conditions of the Glacial 

 period became established, whilst the forms proper to temperate 

 seas receded. As regards the readily locomotive Mammals, 

 also, it is probable that this process was carried on repeatedly 

 in a partial manner, the southern and northern forms alternately 

 fluctuating backwards and forwards over the same area, in ac- 

 cordance with the fluctuations of temperature which have been 

 shown by Mr. James Geikie to have characterized the Glacial 

 period as a whole. We can thus readily account for the inter- 

 mixture which is sometimes found of northern and southern types 

 of Mammalia in the same deposits, or in deposits apparently 

 synchronous, and within a single district. Lastly, at the final 

 close of the arctic cold of the Glacial period, and the re-estab- 

 lishment of temperate conditions over the northern hemisphere, 

 a reversal of the original process took place the northern 

 Mammals retiring within their ancient limits, and the southern 

 forms pressing northwards and reoccupying their original 

 domains. 



