ANCIENT CLIMATES 285 



described 1,200 miles in an hour instead of 1,000 as 

 at present. 



This increased rapidity of rotation would have affected 

 the distribution and velocity of the winds, consequently of 

 sea drift and ocean currents, and therefore of the floating 

 organisms they bear along. 



Still greater would appear to have been the modifica- 

 tions which have affected the past climates of the globe, 

 though these, unlike the spin of the earth, were not always 

 in one direction, but seem to have followed a mighty 

 rhythm, Three times at least the temperature of our 

 planet would seem to have fallen so low that glacial con- 

 ditions intervened within the temperate zone : once at the 

 beginning of the Cambrian ; again, and more evidently, 

 during the Permian period : and yet once again, in times 

 comparatively near our own, when the Great Ice Age held 

 the temperate regions in its grasp, and even touched the 

 high mountains of central Africa. It is indeed doubtful 

 whether we can speak of this interval as wholly past. 

 Three times at least has the temperature varied in the 

 opposite direction, rising so high as to bring the existing 

 climate of the tropics into the temperate zones ; as in the 

 Palaeozoic seras, when coral reefs and coal forests could 

 flourish within the Arctic circle, as again during the 

 Mesozoic periods, and one more finally in the Cainozoic 

 sera, when we can trace the rise of temperature as it 

 follows a curve, making a steep ascent to its culmination 

 in the Eocene, and then slowly and gradually descend- 

 ing till it terminates in a series of oscillations during 

 the last glacial epoch (Fig. 83). 



Such changes surely cannot have been without their 

 effect on the evolution of living forms, and may also 

 have indirectly influenced their distribution. Genial 

 climates would remove many barriers to dispersal, and 

 they would be accompanied by a more active atmospheric 



