8 
ZOOLOGICAL GEOGRAPHY. 
[paet in. 
birds which, though comprising nearly three-fourths of the 
entire class, yet presents hardly any well-marked differences 
of structure by which it can be subdivided — the families confined 
to America are, for the most part, more closely related to each 
other than to the Old World groups. The ten families forming 
the group of “Pormicaroid Passeres,” in our arrangement (vol. 
i., p. 94), are characterised by the absence of singing muscles in 
the larynx, and also by an unusual development of the first primary 
quill ; and seven of this series of families (which are considered 
to be less perfectly developed than the great mass of Old World 
passeres) are exclusively American, the three belonging to the 
Eastern hemisphere being of small extent. Another group of 
ten families — our “ Tanagroid Passeres/’ are characterised by the 
abortion or very rudimentary condition of the first quill ; and of 
these, five are exclusively American, and have numerous genera 
and species, while only two are non-American, and these are of 
small extent. On the other hand the “ Turdoid Passeres,” con- 
sisting of 23 families and comprising all the true “ singing-birds,” 
is poorly represented in America ; no family being exclusively 
Neotropical, and only three being at all fully represented in South 
America, though they comprise the great mass of the Old World 
passeres. These peculiarities, which group together whole series 
of families of American birds, point to early separation and long 
isolation, no less surely than the more remarkable structural 
divergences presented by the Neotropical mammalia. 
In the Picarise, we have first, the toucans (Ehamphastidse) ; 
an extraordinary and beautiful family, whose enormous gaily- 
coloured bills and long feathered tongues, separate them widely 
from all other birds. The Galbulidse or jacamars, the motmots 
(Momotidse), and the curious little todies (Todidee) of the 
Antilles, are also isolated groups. But most remarkable of all 
is the wonderful family of the humming-birds, which ranges 
over all America from Tierra del Euego to Sitka, and from the 
level plains of the Amazon to above the snow-line on the Andes ; 
which abounds both in genera, species, and individuals, and is 
yet strictly confined to this continent alone ! How vast must 
have been the time required to develop those beautiful and 
