SUN AND CLIMATIC VARIATION 53 



These periods of alternate invasion and retreat of the glaciers 

 are called glacial and inter-glacial periods, and have helped 

 to mark the stages of man's history. At that time the /Egean 

 communicated with the Black Sea. After an initial lowering of 

 temperature in the Pliocene, the climate became less rigorous : 

 the Hippopotamus, Rhinoceros, Elephant, Lion, and Hyena, 

 all of them animals to-day confined to tropical areas, flourished 

 in southern France. The glaciers, however, once more gained 

 the ascendancy (the Munsterian age), and spread over one- 

 seventh of the whole land area of the globe, a total of twenty 

 to twenty-five million square kilometres. In the United States 

 they reached 40 lat., somewhere in the neighbourhood of 

 New York ; in Europe they extended as far as lat. 50 , and 

 covered England, North Germany, Scandinavia, and Russia 

 up to the Vorona. The prehistoric x Mammoth and Rhinoceros, 

 whose descendants to-day are practically hairless, were then 

 provided with a coat of woolly hair. When the ice retreated 

 Lemmings were to be found near the glaciers, and further 

 southwards Reindeer, arctic Hare, and arctic Fox. In fact, the 

 fauna of the Steppes reached as far as the Pyrenees. The 

 temperature subsequently became somewhat warmer and the 

 air drier (the climate of the tundras). This was the age of the 

 reindeer, which in the period following was temporarily to be 

 driven northwards, to reappear at the end of the Pleistocene 

 and again to descend to lat. 43 . 



To what are we to ascribe these variations of temperature 

 that brought about four successive glacial invasions ? 

 Formerly, when it was generally assumed that there had been 

 only one glacial period, the explanation of Croll and Geikie 

 sufficed which postulated a gradual lengthening and flattening 

 of the earth's orbit. But the existence of several glacial periods 

 renders this hypothesis of little avail as an explanation. It is 

 also contradicted by the fact that the glacial period would thus 

 seem to have made its appearance simultaneously in both 

 hemispheres. A periodicity connected with sun spots has been 

 suggested, but the period of duration of such spots is far too 

 short, and it is therefore probable that local phenomena were 

 entirely responsible. 



Local phenomena, however, have played but a secondary 

 r61e in the earth's history, and then only for short periods. 



1 Middle Pleistocene or Magdalenian. 



