of Life, front tl/r Xoiili Southward. o9 



Anu'do ETeilprin, that the Sonoran Region is itself a 'Transition 

 Rea-ion ' between the Boreal and Tropical Faunas and Floras. 

 The incorrectness of this hypothesis is easily demonstrated, for 

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

 ture of Boreal and Tropical forms. The contrary has just been 

 shown to be the case, the hiatus between the Sonoran and Boreal 

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

 being not only immense, but vastly greater than that between 

 Boreal America and Eurasia. 



DIFFERENTIATION OF LIFE FROM THE NORTH SOUTHWARD. 



Animals and plants inhabiting the Arctic regions are usually 

 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 many 

 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 powerful influence upon the various forms of life, 

 producing first locfil geographic 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 

 UK tritium*) has no very near relative, and is replaced in the tun- 

 dras by the brown and barren-ground bears (Ursus arclos and 

 richardsoni), which run into several more or less distinct forms, 

 as the snow bear (U. isabellimis) , Syrian bear (U. syriacus), and 

 hairy-eared bear (U. piscator']. Besides these are the grizzly (U. 

 Jion'ibiiiSj of which two forms may be recognized) and the black 

 bears of America and Eurasia (T. amcricanus, torquatus, and 

 j<il>oiucus) ; 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, by the red foxes of America and Eurasia, of which several 

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



