316 Dana on Parallel Relations of the Classes of Vertebrates, 
The discovery of the Reptilian Birds has brought the general 
law to view, that, among the four classes of Vertebrates, ordina- 
may be added in behalf of these four classes. In order to ap- 
preciate their true value, it is necessary to have in view the 
type-idea which is the basis of the fundamental characteristics 
of each, and which is connected with the existence of three 
tinct habitats for life—the water, the air, and the land: that in 
Fishes, this idea is that of cpsapeta' aquatic life; in Reptil 
that of creeping terrestrial life; in Birds, that of flying aerial life ; 
in Mammals, that of terrestrial life, again, mnie in connection wit 
a higher g grade of structure, the Mammalian. e type-idea is 
expressed in the adults both of the typical and hemitypic 
groups; and any attempt to elevate the hemitypic into a sepa- 
rate class tends to obscure these ide al relations of the groups in 
the natural system of Vertebrates. 
The following are the illustrations of the law above mentioned. 
(1.) In the classification of Vertebrates, Mammals, the first 
class, are followed by Birds, as the second ; ‘and while the former 
are viviparous, the latter are, without exception, oviparous. The 
species of the inferior or hemity pic group of Mammals, rtake, 
ec tt, in some degree, of an -aiperavtere: nature, as the term 
