ANCESTRAL MAN. 1 5 



We*have seen that it is i, a causa vera, or a cause in actual 

 existence and operation. 2. It is an efficient cause, occasioning 

 a certain measurable change of climate. 3. It is a reciprocating 

 cause, producing heat by one sweep of its power, and cold by 

 another. 4. It is a prime or initial cause, standing first in a 

 linked series of causal agencies. And 5, it is a cumulative or 

 accelerating cause, bringing about its results, not all at once, but 

 by the gradual sum of a multitude of increments. 



We may suppose, therefore, that whenever the eccentricity of 

 the earth's orbit approached its superior limit, there arose 

 conditions favourable to the occurrence of glacial periods lasting for 

 thousands of years, alternated with equally prolonged periods of 

 genial climate when northern winters happened in perihelion. 

 When the cold was at its greatest, arctic animals like the musk- 

 sheep and the reindeer would range down to the Mediterranean 

 Sea ; and when the cold was at its least, the spotted hysena, 

 the hippopotamus, and other tropical animals would roam to the 

 northern shores of Europe. Thus it is that the bones of creatures 

 of such opposite fauna are sometimes found in the same cave ; 

 and that arctic as well as tropical shells and plant-remains occur 

 in alternating beds of the same locality. 



Moreover, according to the theory, the greatest oscillations of 

 sea level would take place during the severest part of the glacial 

 epoch, when the eccentrcity of the earth's orbit would be at its 

 highest value. These oscillations would gradually diminish in 

 amplitude as the climate gradually became less severe. Now, as 

 a matter of fact, the great subsidence of land as well as its great 

 elevation, or continental ice-period, occurred during the earlier 

 and more severe part of the last glacial epoch ; and as the climate 

 grew warmer these changes of level became smaller, till, as Mr. 

 Croll says, we find them terminating in our submerged forests on 

 the one hand, and our twenty-five-foot raised beach on the other. 



It is of-course difficult to say when the first ice age took place, 

 but there is strong evidence to show that a severe glaciation 

 occurred even in Palaeozoic times. Along the western margin of 

 Sutherland and Ross the Laurentian gneiss presents itself in the 

 form of bosses, hummocks and ridges, which suggest the effects 

 of moving ice ; and these mammillations are accompanied by the 

 smoothing, polishing and striation which tell unmistakeably of 

 glacial action. That these appearances were not recently 

 produced is shown by the fact that Prof. Geikie, confirmed by 

 Prof. Ramsay, was able to trace the rounded surfaces of the 

 Laurentian gneiss passing distinctly beneath the overlying Cam- 

 brian rocks. 



To the period of eccentricity, however, which occurred 

 2,500,000 years ago are referred the glacial possibilities of the 



