66 



ISLAND LIFE. 



[pAirr I. 



central plains and the vast ranges of the Rocky Mountains and 

 Sierra Nevada, that we can hardly expect to find species whose 

 areas may be divided maintaining their identity. Towards the 

 north however the above-named barriers disappear, the forests 

 being almost continuous from east to w^est, while the mountain 

 range is broken up by passes and valleys. It thus happens 

 that most species of birds which inhabit both the eastern and 

 western coasts of the North American continent have main- 

 tained their continuity towards the north, while even when 

 differentiated into two or more allied species their areas are 

 often conterminous or overlapping. 



Almost the only bird that seems to have a really discon- 

 tinuous range is the species of wren, Thryotliorus hewickii, of 

 which the type form ranges from the east coast to Kansas and 

 Minnesota, while a longer-billed variety is found in the wooded 

 parts of California and as far north as Puget Sound. If this 

 really represents the range of the species there remains a gap 

 of about 1,000 miles between its two disconnected areas. Other 

 cases are those of the greenlet, Vireosylvia gilviis, of the Eastern 

 States, and its variety, V. swainsonii, of the Western ; and of 

 the purple red-finch, Carpodacus jpurpicreits, with its variety C. 

 calif ornims ; but unfortunately the exact limits of these varieties 

 are in neither case known, and though each one is characteristic 

 of its own province, it is possible they may somewhere become 

 conterminous, though in the case of the red-finches this does 

 not seem likely to be the fact. 



In a later chapter we shall have to point out some remark- 

 able cases of this kind where one portion of the species inhabits 

 an island ; but the facts now given are sufficient to prove that 

 the discontinuity of the area occupied by a single homogeneous 

 species, by two varieties of a species, by two well-marked sub- 

 species, and by two closely allied but distinct species, are all 

 different phases of one phenomenon — the decay of ill-adapted, 

 and their replacement by better-adapted forms, under the 

 pressure of a change of conditions either physical or organic. 

 We may now proceed with our sketch of the mode of distribution 

 of higher groups. 



Distribution and Antiqicity of Families. — Just as genera are 



