66 
ISLAND LIFE. 
[part 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 west, 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, Thryothorus bewickii, 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 gilvus, of the Eastern 
States, and its variety, V. swainsonii, of the Western; and of 
the purple red-finch, Carpodacus purpureus, with its variety C. 
calif ornicus ; 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 Antiquity of Families. — Just as genera are 
