almost continuous circumpolar land; and that these plants and animals, both in the 

 Old and New Worlds, began slowly to migrate southwards as the climate became less 

 warm, long before the commencement of the Glacial period." By the cooling during 

 later Pliocene times, he explains, these species became isolated and modified, so 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 shown that 



more plants are identical than was formerly supposed), but we find a host of closely 



allied representative forms." 



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