150 ON THE CLASSIFICATION OF BIRDS. 
the humming-birds, we have already shown that they 
form among themselves a circular group ; it conse- 
quently follows that the genera by which they are 
connected with the hoopoes must, if in existence, 
belong to the Promeropidce, — a family wherein the 
species and forms are few, and where there is a 
sufficient interval to admit of two or three intermediate 
genera, the discovery of which would at once fill up 
the hiatus between Promerops and Phosthornis. Hav- 
ing already offered several remarks on the relations 
of this tribe*, when compared to other ornithological 
circles, we may now at once pass on to the last of 
the principal groups in the order. 
CHAP. XI. 
ON THE TRIBE OF FISSIROSTRES, OR SWALLOW-LIKE BIRDS. 
(169.) The order of Fissirostbes is best represented 
by the swallows and goatsuckers : the former are the 
most isolated; while the latter, above all other birds, 
show the nearest affinity to the owls. No species, 
indeed, has yet been discovered, which would perplex 
a naturalist to decide to which of these families it 
belonged : but that is not material : we do not uphold 
the injudicious theory that every one of nature’s links 
are so perfect, or rather, are so well known, as to leave 
no unequal intervals in the series ; on the contrary, we 
maintain that such interruptions are frequently found, 
and, in this manner, are the goatsuckers detached by a 
slight interval from tile owls. 
(170.) The flssirostral birds, as a whole, are 
peculiarly distinguished, by having the powers of flight 
developed in the highest degree : all the energies of 
their nature seem concentrated in this one perfection ; 
* North. Zool. 
