68 ISLAND LIFE part i 



species their areas are often conterminous or over- 

 lapping. 



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

 tinuous range is the species of wren, Thryotliorus hcwicJdi, 

 of which the type form ranges from the east coast to 

 Kansas and Minnesota, while a longer-billed variety, 

 T. leioickii sioilurus, is found in the wooded parts of 

 California and as fa5r 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 Vireo hellii of the middle United 

 States and the sub-species intsillus of California ; and of 

 the purple red-finch, Carjpodacusinirpureus, with its variety 

 C. califo7micus ; 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 that 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 re- 

 markable 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 phenome- 

 non — 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. 



DistrilmtiGn and Antiquity of Families. — Just as genera 

 are groups of allied species distinguished from all other 

 groups by some well-marked structural characters, so 

 families are groups of allied genera distinguished by more 

 marked and more important characters, which are generally 

 accompanied by a peculiar outward form and style of 

 colouration, and by distinctive habits and mode of life. 

 As a genus is usually more ancient than any of the species 

 of which it is composed, because during its growth and de- 

 velopment the original rudimentary species becomes sup- 



