64 EVOLUTION OF LIFE. 
climbing (woodpecker), birds of prey (Fig. 77) (eagle), 
and running birds (Fig. 73) (ostrich). This classification is 
only useful as a means of superficially arranging the eight 
thousand species of birds known to naturalists. An ex. 
amination of the skeleton, however, offers us only three 
orders, the first of which, possessing a long bony tail, is 
represented by a single genus, the extinct Archeopteryx ; 
the second, in which the breast-bone is furnished With a 
ridge or keel, whence the name Carinata, includes ordinary 
birds; the third order has no keel on its breast-bone, and 
is represented by the Ostrich, Emeu, etc. Although birds 
differ considerably from reptiles, yet they agree with, one 
another in very many important characters, such as peculi- 
arities in the structure and articulation of the skull and 
backbone, and in the structure and articulation of the lower 
jaw with the skull. The brain, heart, and great cavities 
of the body are alike in these orders. These common 
characters, às well as others, seem to warrant the union of 
birds and reptiles in a natural group, of which the birds 
are the most advanced in specialized structure. This view 
is confirmed by the embryology and fossil remains of birds 
and reptiles. The Ostrich family, among existing birds, 
perhaps approaches nearest the reptiles. Nevertheless, 
the gap was very great between these classes. By the dis- 
covery of the existence in past time of the Dinosaurian 
reptiles and Archeopteryx, this gap has been bridged over. 
The Archeopteryx offers the only known example of a bird 
with a bony tail, that is, among adult birds, as birds, when 
in an embryonic condition, have quite as much of a tail 
as a turtle. The bones of the hand (metacarpal), in the 
Archeopteryx, are not soldered together (anchylosed) 
as in birds, but remain distinct. In these marked and 
important peculiarities the Archeopteryx agrees with rep- 
tiles and differs from birds. The Archeopteryx, in fact, is 
a reptile-like bird. In the Compsognathus, a fossil reptile, 
