158 ALTEENATE GLACIAL PEKIODS [Chap. XIL 



As on tlie 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 

 Ciixle, will account, on the theory of modification, for 

 many closely allied forms now living in marine areas 

 completely sundered. Thus, I think, we can under- 

 stand 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 con- 

 tinent 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 Porbes's view may be largely 



