58 
THE COLUMBIDiE. 
truly Gallinaceous) are also monogamous.—Again, he says,““ No doubt there 
is some similarity in the sterna, but it is confined to the posterior angles, and 
consists in there being two notches in each.” Now I deny that there is any thing 
that can be fairly called similarity between the sterna; the Gallinaceous birds 
have two deep notches or emarginations extending from the posterior edge of the 
sternum more than two-thirds of its length. But few of the Columhidcje have two 
notches, and where that is the case, the external one extends little more than 
half the length of the sternum, and the internal one not more than the eighth 
part of its length; but in most of the Columbidce two small holes or foranhna 
supply the place of the internal notches. 
I think that each family ought to possess several distinct points of resemblance, 
in habits of life, functions, external appearance, and internal structure, with 
every other family of the same group, or at least with the typical genera of that 
group, which no other group of birds possesses in common with them; that each 
genus should consist of birds similar in their general habits, appearajice, and 
structure, but constantly differing from each other in some one character; and 
that this constant difference in some one particular constitutes a species. When 
there is no general resemblance through the whole of the families, but only 
a similarity in a few particulars between some of the extreme species of each 
family, they ought not to be included in the same order, this connection 
between the extremes of the different families, clearly pointing out that they 
should be considered as distinct consecutive orders. 
I will now enumerate the points of difference and agreement between what I 
consider the true Gallinaceous birds (^. e. the Pavonidce^ Tetraonidce.y and Cracidm 
of Selby) and the Columbidce* 
In my comparison between the Columbidce and Rasores, I do not include 
Selby’s Geophilus, This genus is little known. It appears to bear a much 
nearer resemblance to the Rasores than any other of the Pigeon family, particu¬ 
larly as regards the number of eggs; the young being able to see at birth; being 
hatched on the ground, and running as soon as hatched; but I think all these 
points of resemblance afford no reason why all the Pigeons should be classed with 
the Rasores; but that they only prove a connecting link to exist between them. 
Pigeons—with the exception of the Rock Pigeon—build their nests in trees. 
The Rasores —with the exception of some species of Cracidce —uniformly make 
their nests on the ground. 
Pigeons never lay more than two eggs.—The Gallinaceous birds always lay 
more than two. 
Pigeons are blind at birth, and remain so several days.—The Gallinaceous birds 
see at birth. 
Pigeons remain several weeks in the nest after they are hatched, and until 
