184 ON THE CLASSIFICATION OP BIRDS. 
itioiis of cold and heat. As the ocean is their element, 
so do its waters supply their food : some live both 
upon aquatic plants and submarine insects ; hut the 
greatest proportion prey upon fish, and those innumer- 
able swimming and creeping things which subsist in 
the sea, and cover its shores. 
(205.) The aquatic order has been divided into the 
following families *, which have every appearance of 
being the natural primary divisions: — 1. The Ducks 
(Anatid^) ; — 2. The GuUs (Laridas) ; — 3. The 
Pelicans (Pelicanidas) ; — 4. The Penguins (Alci- 
DAi); — 5. The Divers (Colymbida:). The three first 
are distinguished by the length of the wings, which en- 
able them to fly well ; while, in the two latter, these 
members are so short, that they seem perfectly useless 
for any other purpose than to serve the office of fins. A 
rapid survey of the prominent characters and groups of 
these families is all that can be here said of them. 
(206.) The analogies of the primary groups of the 
natatorial order have never been made out, and they 
are, in truth, involved in much difficulty ; at least, if 
we are to judge from the trouble their investigation has 
cost us. There can be no doubt, indeed, that the 
typical perfection of the order rests with those birds 
whose structure is more especially adapted for diving 
and swimming ; and it therefore follows, as the result 
of this conclusion, that the Cnlymbidce, or divers, and 
the Alcida, or auks, constitute the two primary types : 
the lengthened and conic bill of the first of these 
families assimilates with that form belonging to the 
ConiroHres; and we may, therefore, safely infer that 
it represents that tribe. The Alcidm, on the other 
hand, have a short and broad bill, considerably curved 
on the culmen, where it is gradually arched from the 
base ; while the tip, although not toothed, is bent over 
the lower mandible : this, in fact, is the general outline 
of the bill of dentirostral types, even in remote groups, 
such as the Psittucidfej among the Scansoves ,* and it is 
* Vigors, Linn. Trans, xiv. 
