376 



THE AGE OF MAMMALS 



1. Geological: glacial deposits and erosions. 



2. Botanical : plant deposits, alternation of northern, arctic, and tem- 

 perate floras. 



3. Pala3ozoological : evolution, migration, and extinction of animal life. 



4. Archaeological : human implements, stages of human culture, skeletal 

 remains of man. 



Geologists are agreed that there were several glacial advances, differing 

 in duration and severity, alternating Avith inier glacial warm periods during 

 which the ice retreated and conditions of climate prevailed which in some 



1 

 ^GLACIAL 



Fig. 173. — Europe in glacial times. I. Maximum glaciation. After de Lapparent, 1906. 



instances were even milder than those of the same latitudes to-day. While 

 in a sense rhythmic, both the glacial and interglacial periods differed in 

 duration; they also differed in intensity in different parts of the northern 

 hemisphere and as affected by the proximity of great mountain ranges or 

 highlands on which the ice masses could accumulate. Thus near the Swiss 

 Alps, Penck (1909) observes evidence of four great glacial advances and three 

 interglacial epochs, while in northern Germany only three great glacial 

 advances are recorded, because one of the series was not sufficiently extensive 



