THE POLAR GLACIERS. 709 



form by the combined attractions of the other planets, Jupiter carry- 

 ing the controlling influence. When the average of all these forces 

 for long periods is more in one direction than in another, our planet is 

 drawn away from the sun on that side. Xow, it must occasionally 

 happen, with the various periods of revolution of the planets, that 

 they unite at times to produce extreme irregularities. The present 

 diflerence between the nearest and farthest distance of the sun from 

 us is 3,200,000 miles. It is found, by calculating back the planetary 

 orbits and conjunctions, that this focal distance has been as much as 

 14,000,000 miles. There was, then, an excess of thirty-nine winter 

 days during each year of the great secular winter of either pole. 

 This excej^tionally high eccentricity occurred, according to the calcu- 

 lations of Mr. James Croll, about 850,000 years ago. But it is now 

 generally thought that we have no need to go back as far as that for 

 the period of the last glacial epoch : 200,000 years ago the focal dis- 

 tance was 10,500,000 miles, and the winter excess twenty-eight days. 

 This, on the supposition heretofore made of the absolute zero of cold 

 being at least 257 below the freezing-point, would lower the mean 

 temperature in polar regions 50 Fahr,, and would unquestionably ex- 

 tend the permanent ice-limits far into the temperate zone. From that 

 time, down to 70,000 years ago, the eccentricity was continually from 

 two to four times greater than now. Since about 70,000 years ago, it 

 has been nearly all the time less than at present. Thus it may fairly be 

 concluded that the great glacial period of the Post-tertiary era came 

 to an end with the fourth secular winter in the past, or b, c. 67,000. 



This is a very interesting date to us of the genus Aow^o/ for it 

 must have been about this time, according to all accounts, that our 

 forefathers made their appearance on the earth. Man, with the long- 

 haired mammoth, the woolly rhinoceros, the huge cave-bear, the great 

 horned reindeer, and numerous other species now extinct, followed 

 close upon the retreating ice-fields of the bowlder period. Our prime- 

 val ancestors were a race of hunters, and they subsisted on the most 

 abundant and magnificent game that the world has ever seen. They 

 lived in caves or under projecting ledges, and with only flint-headed 

 weapons contested their lives and homes with savage beasts. They 

 cracked the bones of animals for their marrow, or crushed them in 

 stone mortars for the fats and the juices which they contained. It was 

 the lingering carnivorous instinct to gnaw the bones of their prey. 

 They had fires at their funeral feasts, but there is little evidence of 

 their indulging often in the luxury of cooked meats. It was a rude 

 life, and a hard struggle they must have had for it; but their history 

 is read in the drift-beds and cave-deposits of Europe, as plainly as 

 if there had been an Herodotus to write it. 



The efiect and bearing of the great ice periods on geological work 

 and time will be further considered in a second article in continuation 

 of this. 



