314 ALTERNATE GLACIAL PERIODS [CHAP. XIL 



Europe and N. America. Hence it has come, that when we 

 compare the now living productions of the temperate regions of 

 the New and Old Worlds, we find very few identical species 

 (though Asa Gray has lately shown that more plants are identical 

 than was formerly supposed), but we find in every great class 

 many forms, which some naturalists rank as geographical races, 

 and others as distinct species; and a host of closely allied or 

 representative forms which are ranked by all naturalists as 

 specifically distinct. 



As on the land, so in the waters of the sea, a slow southern 

 migration of a marine fauna, which, during the Pliocene or even a 

 somewhat earlier period, was nearly uniform along the continuous 

 shores of the Polar Circle, will account, on the theory of modifica- 

 tion, for many closely allied forms now living in marine areas 

 completely sundered. Thus, I think, we can understand the 

 presence of some closely allied, still existing and extinct tertiary 

 forms, on the eastern and western shores of temperate North 

 America; and the still more striking fact of many closely allied 

 crustaceans (as described in Dana's admirable work), some fish 

 and other marine animals, inhabiting the Mediterranean and the 

 seas of Japan, these two areas being now completely separated 

 by the breadth of a whole continent and by wide spaces of ocean. 



These cases of close relationship in species either now or 

 formerly inhabiting the seas on the eastern and western shores of 

 North America, the Mediterranean and Japan, and the temperate 

 lands of North America and Europe, are inexplicable on the 

 theory of creation. We cannot maintain that such species have 

 been created alike, in correspondence with the nearly similar 

 physical conditions of the areas ; for if we compare, for instance, 

 certain parts of South America with parts of South Africa or 

 Australia, we see countries closely similar in all their physical 

 conditions, with their inhabitants utterly dissimilar. 



Alternate Glacial Periods in the North and South. 



But we must return to our more immediate subject. I am 

 convinced that Forbes's view may be largely extended. In 

 Europe we meet with the plainest evidence of the Glacial period, 

 from the western shores of Britain to the Oural range, and 

 southward to the Pyrenees. We may infer from the frozen 

 mammals and nature of the mountain vegetation, that Siberia 

 was similarly affected. In the Lebanon, according to Dr. Hooker, 

 perpetual snow formerly covered the central axis, and fed glaciers 

 which rolled 4000 feet down the valleys. The same observer has 

 recently found great moraines at a low level on the Atlas range 

 in N. Africa. Along the Himalaya, at points 900 miles apart, 

 glaciers have left the marks of their former low descent ; and IB 



