320 MAX AND THE GLACIAL PERIOD. 



that Atlantic species would have migrated to the Pacific 

 side and obtained a permanent lodgment there, and that 

 Pacific species would have found a congenial home on the 

 Atlantic side. It must be confessed that this is a serious 

 theoretical difficulty, but perhaps not insuperable. For 

 it is by no means certain that colonists from the heated 

 waters of the Caribbean Sea would become so permanently 

 established upon the Pacific side that they could maintain 

 themselves there upon the re-establishment of former 

 conditions. On the contrary, it seems reasonable to sup- 

 pose that upon the re-elevation of the isthmus the north- 

 ern currents, which would then resume their course, would 

 bring back with them conditions unfavourable to the At- 

 lantic species, and favourable to the competing species 

 which had only temporarily withdrawn from the field, and 

 which might now be better fitted than ever to renew the 

 struggle with their Atlantic competitors. It is by no 

 means certain, therefore, that with the re-establishment 

 of the former conditions there would not also be a re- 

 establishment of the former equation of life upon the two 

 sides of the isthmus. 



Mr. Upham's theory involves also extensive elevations 

 of land in the northern part of America ; in this respect 

 agreeing with the opinions early expressed by Professors 

 J. D. Dana and J. S. Newberry. Of the positive indica- 

 tions of such northward elevations of land we have already 

 spoken when treating in a previous chapter of the fiords 

 and submerged channels which characterise northern 

 Europe and both the eastern and the western coasts of 

 North America. But in working out the problem the 

 solution is only half reached when we have got the Gulf 

 Stream into the Pacific Ocean, and the land in the north- 

 ern part of the continents elevated to some distance above 

 its present level. There is still the difficulty of getting 

 the moisture-laden currents from the Pacific Ocean to 

 carry their burdens over the crest of the Sierra Nevada 



