40 Merriam Geographic Distribution of Life. 



species, and third, by additional types, at least one of which in 

 our own country is entitled to generic rank (l T rocyon). 



The ermine and polar hare are the sole Arctic representatives 

 of groups which in the temperate parts of Europe and America 

 comprise many distinct species, and in the case of the former, 

 several well marked suhgenera. 



The Arctic lemmings (genera Mi/odes and (.'// trirulux) are nu- 

 merously represented in the north temperate parts of the world 

 by the genets, Ettobius, Synaptoniys, Phenacomys, Evotomys, 7'VAr/ 1 , 

 and Arcicola. 



It is not to be inferred from the above remarks that the polar 

 representatives of these various groups are to be looked upon 

 as the parent stocks from which the other members sprang. 

 Usually the reverse is the case, for groups of Boreal origin that 

 now attain their maximum development in north-temperate 

 regions have their numbers reduced in the Arctic circle to a 

 single representative. But, regardless of centers of origin, it is 

 here intended to emphasize the fact that types inhabiting the 

 Arctic Zone are few in number and uniform in character 

 throughout their distribution, while to the southward the same 

 types become more and more diversified and new types appear 

 as the distance from the Pole increases,* so that it may be 

 formulated as a general proposition that in continental areas 

 the further from the Pole* the larger the Dumber of families, genera, 

 and species, f 



*The elder Agassiz long since pointed out that "the vegetation of the 

 two continents becomes more and more homogeneous the more we 

 advance northward" (Lake Superior, 1850, 153). Stated conversely, this 

 is in complete accord with the "Law of differentiation from the north 

 southward " formulated by Allen as " a constant and accelerated diver- 

 gence in the characters of the animals and plants of successive regions of 

 the continent." (Bull. Mus. Coinp. Zool. II, 1871, 379.) In a later con- 

 tribution the same author speaks of the " high rate of differentiation 

 favored by tropical conditions of climate," and adds that Arctic; and 

 cold-temperate climates are characterized by only slightly or moderately 

 diversified faunas ; that a moderate increase of temperature results in the 

 addition of many new types; and that "a high increase in temperature, 

 giving tropical conditions of climate," is accompanied by "a rapid multi- 

 plication of new forms and a maximum of differentiation." 



fThis is a general proposition intended to apply to terrestrial forms of 

 life collect! r<h/ 7 ami does not conflict with the law that the maximum 

 number of species in each particular group is found in the /one or area 

 which is the center of its distribution. 



