■Differentiation of Life from fl/r XorfJi SoutJnvard. 39 



Angelo Heilprin, that the ^onoran Uujioii is itself a ' Transition 

 Region ' lietween the l^)oreal and Tropical Faunas and Floras. 

 The incorrectness of this livj-uthesis is easily demonstrated, for 

 it rests upon the assumption that the Sonoran Region is a mix- 

 ture of Boi;eal and Tropical forms. The contrary has just l)een 

 shown to 1)e the case, the hiatus between the Sonoran and Boreal 

 on the one hand and the Sonoran and Tropical on the other 

 Ijcing not only immense, l)ut vastly greater than that between 

 Boreal America and Eurasia. 



Differentiation of life from the north southward. 



Animals aud plants inha1)iting the Arctic regions are usuall}^ 

 specifically identical throughout Arctic America, Greenland, and 

 the polar parts of Eurasia and outlying islands, while as they 

 diverge from the pole southward they tend to split up into luany 

 species ; in other words, Boreal species are more stable and per- 

 sistent than those inhabiting warmer countries. The explana- 

 tion of this fact is obvious. The identity of climate and environ- 

 ment throughout the Arctic Zone tends to preserve identity of 

 specific characters, giving rise to a homogeneous fauna and flora, 

 while the diversity of physical conditions and climatic influences 

 prevailing in an increasing degree at greater distances from the 

 pole exerts a ]iowerful influence upon the various forms of life, 

 producing first local geogra])hic races or subspecies, then species, 

 and finally groups of species constituting well-marked subgenera 

 and even genera, giving rise to greatly diversified faunas and 

 floras. Thus among mammals the polar or ice bear ( Thalarctos 

 maritimux) has no ver}^ near relative, and is replaced in the tun- 

 dras by the l^rown and l)arren-grouud bears ( Ursus nrclos and 

 ^richardmni) , which run into several more or less distinct forms, 

 as the snow bear {U. isaheUinvs), Syrian bear (U. syrincus), and 

 hair3^-eared l^ear (U. piscator). Besides these are the grizzly (U. 

 horribilis, of which two ibrms may be recognized) and the Ijlack 

 bears of America and i^urasia {U. americanus, torqaatu><, and 

 japoniras) ; and still further southward the group becomes dif- 

 ferentiated into several well-marked genera. 



In like manner the Arctic fox is replaced to the southward, 

 first, l)y the red foxes of America and Eurasia, of which several 

 subspecies are known: second, liy a number of quite distinct 



