FOSSIL MAN. 619 



period when winter in the Northern hemisphere coincides with 

 the earth's position in its orbit farthest from the sun, the mean 

 temperature of the Northern hemisphere will be considerably 

 lower than when the reverse conditions prevail that is, when 

 winter in the north coincides with the earth's position in its orbit 

 nearest to the sun, as at present. Moreover, this change of posi- 

 tion of the earth in its orbit would likely result in a change in 

 direction of the trade winds, the air currents being mainly caused 

 by the expansive action of heat creating a vacuum into which 

 the air rushes from colder areas ; therefore, as the Southern hemi- 

 sphere would become the warmer, the prevailing winds would be 

 southward, thus changing the direction of ocean currents, like 

 the Gulf Stream, to the southward, this being a secondary cause 

 resulting from the first and further intensifying the cold. 



This hypothesis is supported by the fact that at present win- 

 ter in the Southern hemisphere coincides with the earth's position 

 in its orbit farthest from the sun, and the ice around the south 

 pole extends much farther toward the equator than that around 

 the north pole, the antarctic ice extending as far as the sixty- 

 seventh degree of latitude. The antarctic summers are also said 

 to be more humid, cold, and chilly than the arctic summers. 



But the earth's orbit is not always the same ; there have been 

 periods when the orbit described resulted in a considerably greater 

 difference than three million miles, and it is thought that if this 

 ordinary eccentricity of the orbit is insufficient to account for 

 the Glacial epoch, the periods of greater eccentricity would. 



This hypothesis seems to explain rather overmuch, for if true, 

 it accounts not only for the Glacial period, but for many glacial 

 periods in the past, for, other conditions remaining the same, 

 there would result a Glacial period for every period in the past 

 that the earth held the proper relative position in its orbit ; and if 

 true, there should be geological evidence to sustain it, which there 

 does not appear to be, for such evidence of earlier glaciers as the 

 rocks of past geological ages exhibit would seem to indicate local 

 glaciers, not any widespread glacial action ; but it would be rash 

 to maintain that the other conditions remained the same ; geology 

 can not be said to show that they did. 



Still another hypothesis is that the Isthmus of Panama was in 

 glacial times submerged, thus allowing the Gulf Stream to fiow 

 into the Pacific Ocean and thence north ; also that the northern 

 coast of British America was more elevated than at present. 

 There is geological evidence to sustain both these propositions, 

 but not conclusively. It is obvious that under these conditions 

 the eastern part of the northern coast would be much colder, 

 being more elevated and having lost the heat emanating from the 

 Gulf Stream, while the western part would be warmer, having 



