THE GLACIAL PERIOD. 255 



10,500 years, and then the original condition of things will 

 be restored. The ice will go back to the North, the genial 

 phase will again gladden llie South. In fact, so long as the 

 eccentricity remains high there will be an oscillation of ice 

 from the North to the South and back again with a corre- 

 sjionding interchange of the genial conditions. How many 

 times this may happen at any particular epoch of maximum 

 eccentricity I do not now attempt to say. It is, however, 

 certain that the gradual return of the earth's orbit to a 

 nearly circular shape will prevent its indefinite repetition. 

 After the lapse of hundreds of thousands of years the orbit 

 of the earth again elongates to an oval, and a recurrence of 

 the phenomena that I have just described takes place. 



The conclusion to which the astronomical theory leads us 

 is that in the course of time there must have been several 

 occasions on which the necessary condition of glaciation were 

 found to concur. The dates of the several ice ages must 

 have been very irregular, there being, however, a certain 

 tendency among them to cluster together in groups, sepa- 

 rated by the comparatively brief interval indicated jiy the 

 precession of the equinoxes. It is, however, essential for me 

 to add that the severity of the different ice ages must have 

 shown many varieties, graduating from the most intense 

 down to a condition hardly differing from the climates with 

 which we are familiar. 



I have endeavoured to assign its true ])roportions to the 

 astronomical factor which has, I believe, been a potent 

 influence in the climatic changes of our globe ; but it must 

 not be understood that I overlook the geological agents 

 which have undoubtedly contributed in no small degree to 

 the modification of climate. It will be here sufficient to 

 mention a single fact to illustrate how the results that would 

 be brought about by the opei'ation of purely astronomical 

 causes are often altered by terrestrial conditions. The fact 

 that 63 per cent, of the heat on our hemisjjliere comes in 

 sumfJer, and 37 ])er cent, in winter, Avould seem to require a 

 vast difference between the winter temperature and the 

 summer temperature even with nearly equal seasons as we 

 have at present. But in the British Isles or in Tasmania 

 the proximity of the ocean has such a moderating effect on 

 temperature that the range between summer and winter 

 is greatly reduced. To find normal temperatures so flir as 

 the astronomical conditions are involved, we must look to 

 some locality where the moderating elements are insignificant. 



