PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 377 



limited portions of a continent. Xot a few range over tlie greater 

 part of whole hemispheres, while by far the larger number are con- 

 fined within comparatively narrow limits. Of the numerous species 

 of mammals and birds inhabiting Xorth America, none are equally 

 common throughout the whole extent of the continent. The habitats 

 of a few only extend from the Barren Grounds of the Arctic regions 

 to Mexico, and from the Athmtic coast westward to the Pacific; one 

 or two only among the mammals range over the whole continent from 

 Alaska to Central America, while some occupy merely the extreme 

 boreal parts of the continent. The latter, in many cases, range also 

 over the Arctic and sub-Arctic regions of the Old World. Others 

 extend from Arctic America southward to the United States. Still 

 others occupy only the middle or more temperate latitudes, being un- 

 represented in the extreme north or the extreme south. Others, 

 again, first appear in the middle or more southerly parts, and range 

 thence southward far into the Tropics. A largenumber are restricted 

 to the region east of the Rocky Mountains; others are confined to a 

 narrow belt along the Pacific coast; and others still to limited areas 

 of the great Rocky Mountain Plateau. In general, their distribution 

 accords with climatal regions or zcmes, their respective ranges being 

 limited in part by latitude and in part by geographical barriers, as 

 treeless, arid plains, or high mountain ranges. The northern and 

 southern boundaries of the habitat of a species are found to agree, not 

 generally with the arbitrary parallels of the geographer, but with 

 isothermal lines, these being more or less different for each species. 

 The geographical distribution of a species is thus mainly determined 

 by climatic or other physical causes, though in part, doubtless, by its 

 organic constitution. In most cases species that are wide ranging 

 are the most variable, as would naturally follow from their being sub- 

 jected, in the different portions of their habitats, to widely different 

 environing circumstances. 



Hence such species are often found to run into numerous local races, 

 some of them greatly differing from others, but still inseparably 

 connected by indiA^duals inhabiting the intervening regions. Over dis- 

 tricts slightly diversified, even if of large extent, species generally pre- 

 serve comparative constancy of character, while, conversely, local races 

 are of frequent occurrence in regions of alternating valleys, mountain 

 ranges, and table lands, and more especially is this true if the highly 

 diversified region be situated in the warmer latitudes. Small islands, 

 remotely situated from other lands, have usually many species pe- 

 culiar to themselves, their differentiation being proportionate to the 

 geologic antiquity of the islands and their remoteness from larger 

 land areas. In islands of recent origin and not widely separated 

 from continental lands, the ancestral stock of the species is still often 

 SM 1905 28 



