EXCURSIONS OF AN EVOLUTIONIST 



have seen that it is certain that men inhabited 

 Britain contemporaneously with the big-nosed 

 rhinoceros, which became extinct about the be- 

 ginning of the Glacial period. How long men 

 lived upon the earth before that time we do not 

 know ; but it is clearly established that there 

 were men in Britain then. It would accordingly 

 be very interesting to know when the Glacial 

 period began to come on in Europe. But on 

 this point it has already become possible to 

 form something like a definite opinion. 



To understand how we can arrive at a date 

 for the Glacial period, it is necessary first to 

 understand the cause of that wonderful change 

 of climate which allowed all Europe as far 

 south as Dresden, and all America as far south 

 as Philadelphia, to become swathed in an ice- 

 sheet like that which now covers Greenland. 

 The causes of this event were many and com- 

 plicated, but the arch-cause the cause which 

 unlocked all the others and set them going 

 was an astronomical cause. It has been proved 

 by Mr. Croll that the primary cause of the gla- 

 ciation of the northern hemisphere was a change 

 in the shape of the earth's orbit, such as had 

 occurred before and will occur again ; and the 

 dates of these changes in the orbit, whether past 

 or future, can be determined by astronomical 

 methods with great accuracy. 



The reason why the weather is warmer in 

 48 



