GEOGRAPHIC DISTRIBUTION OF LIFE IN NORTH AMERICA. 395 
these are the grizzly (U. horribilis), of which two forms may be recog- 
nized) and the black bears of America and Eurasia (U. americanus, 
torquatus, and japonicus); and still farther southward the group be- 
comes differentiated 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 sub-species are 
known; second, by a number of quite distinct species; and third, by 
additional types, at least one of which in our country is entitled to 
generic rank (Urocyon). 
The ermine and polar hare are the sole Arctic representatives of 
groups which in the temperate parts of Kurope and America comprise 
many distinct species, and in the case of the former, several well- 
marked sub-genera. 
The Arctic lemmings (genera Myodes and Cunieulus) are numerously 
represented in the north temperate parts of the world by the genera 
Ellobius, Synaptomys, Phenacomys, Evotomys, Fiber, and Arvicola. 
It is not to be inferred from the above remarks that the polar repre- 
sentatives 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 the north temperate regions have theirsnumber reduced 
in the Arctic circle to a single representative. But regardless of cen- 
ters of origin, it is here intended to emphasize the fact that types in- 
habiting the Arctic zone are few in number and uniform in character 
throughout their distribution, while to the southward the same types 
become moreand more diversified andnew 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 poles, the 
larger the number of families, genera, and species. t 
* The elder Agassiz long since pointed out that ‘ the vegetation of the two conti- 
nents becomes more and more homogeneous the more we advance northward.” (Lake 
Superior, 1850, 153.) Stated conversely, this is in complete accord aith the ‘ Law 
of differentiation from the north southward” formulated by Allen as ‘a constant 
and accelerated divergence in the characters of the animals and plants of suecessive 
regions of the continent.” (Bull, Mus. Comp. Zool. 11., 1871, 379.) In a later contri- 
bution the same author speaks of the “high rate of differentiation favored by trop- 
ical 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 multiplication of new forms and a maximum of differentiation.” 
t This is a general proposition intended to apply to terrestrial forms of life col- 
lectively, and does not conflict with the law that the maximum number of species in 
each particular group is found in the zone or area which is the center of its distri- 
bution, 
