296 



DATES OF THE VARIATIONS IN 



[Ch. XIII 



difficult 



mi 



when man co-existed with a great many species of mammalia 

 now extinct, and when the caves containing the bones of 

 such animals as well as human remains bore a relation 

 to the drainage of the country and levels of the valleys, 

 differing widely from the state of things now established. 

 We may . also conceive the migration of quadrupeds from 

 northern and southern provinces into the basins of the Seine 



* 



and Thames to have taken place during the alternate warmer 

 and colder phases of precession, when the climate was under 

 the influence of an excentricity nearly four times greater than 

 that of our era. The carcasses of the elephant and rhinoceros 

 may then have been enveloped in Siberian ice ; but if so, we 

 must assume that neither the summers nor winters in peri- 

 helion have at any subsequent period had power to melt so 

 much ice as to expose these carcasses to decomposition — a 

 fact which implies a persistent polar ice-cap during each phase 

 of precession. 



When we pass beyond the era B, we find an interval of 

 more than half a million of years without an excentricity so 

 large as A and B before mentioned, although it embraces 

 three periods in which the excentricity was more than double 

 what it now is. In such a lapse of ages, changes in physical 

 geography may have exercised considerable influence in 

 giving rise to oscillations of temperature, even though the 

 proportion of polar land was always in excess. It would, 

 therefore, be very hazardous to exclude all these ages from 

 the Glacial Period, merely on the grounds that such intensity 

 must have coincided with a higher excentricity than is sup- 

 plied by that long interval. If, indeed, we could make such 

 an assumption, the period C would, as Mr. Croll suggests, have 

 the best claim to be the epoch of extreme cold, notwithstanding 

 that the intervening era C b presents us with an excentricity 

 smaller than that of the present times intercalated between 

 Ca, four and a half times greater than the present, and Ce 

 three and a half times greater. 



The whole duration of C might be said to comprise about 

 six complete cycles of precession, because one of them would 





