250 BULLETIN OF THE 



from their special means of locomotion, able to range widely ; but to 

 their allies, the moles and shrews, mountain chains and arid plains prove 

 impassable barriers. 



The same laws in respect to the character of the species that among 

 mammals have a wide distribution are equally exemplified in birds, 

 all the wide-ranging species being of high rank, or such members of 

 lower groups as have the power of flight unusually developed. The 

 modification of the anterior limbs into organs of flight specially charac- 

 terizing the class of birds among vertebrates, it is evident that well-de- 

 veloped wings are one of the elements essential to a high grade of 

 structure ; and this renders necessary the coincidence in this class of 

 high rank with a wide geographical range. The few land-birds that em- 

 brace a large portion of the two northern continents within their respec- 

 tive habitats belong principally to three families, — the finches, and the 

 hawks and owls. The first is one of the highest, if not the highest, 

 family of the class, and the others are by no means low. The other 

 species which have a circumpolar distribution are among the highest 

 members of their respective families, and are rarely of a low grade. 

 The finches thus distributed all belong to the highest genera of their 

 family. Among the birds having a wide distribution, but which are re- 

 stricted to a single continent, are the typical thrushes, another of the 

 higher groups. The species of the short-winged genera of the Fringil- 

 lidae and Turdidae, on the other hand, are almost invariably the most cir- 

 cumscribed in their habitats.* This coincidence in respect to structure 

 and distribution is also exemplified in every sub-family, as well as family, 

 among the water-birds ; but it is not necessary to trace it further here. 



Hence the view above taken in reference to the species claimed to be 

 common to the Old World and the New is supported, not only by the 



* Compare* the species of Tardus with those of Tlarporlujnchus and Mimus; of Poocce- 

 tes and Passt rculus ( see observations on some of the supposed species of Passerculus in 

 Mem. Bost, Soc. Nat. I list., Vol. I, p. r,l5) with those of Melospiza, Cotwniculus, and 

 Ammodron us ; or those of the sub-family Coccotkravstince with those of the sub-family 

 ■ llince. Compare, also, in the Sylvicolida . the species of D< ndi acn with those of Geo- 

 (hhjpis. Also note the very high rank of the species of sEgiolhus, Pinicola, and I 

 trqphanes, and the wide extent of their habitats. Compare further, in Falconida, the 

 species of Falconince, with their long pointed wings and compact firmly knit muscular 

 bodies, giving unequalled powers of flight, and their extensive habitats, in several in- 

 stances embracing a whole hemisphere, with the comparatively short-winged, sluggish, 

 and clumsy species of Bulconince, of a much lower type of structure and much narrower 

 range. 



