320 Dana on Parallel Relations of the Classes of Vertebrates. 
on through the early part, at least, of the life of the chick, or 
until it was fledged. 
This conclusion is made to appear still more reasonable by 
the following comparison of the three obvious methods of sub- 
dividing Vertebrates, and the connection therewith of the char- 
acteristics of the hemitypic groups. These three methods are— 
. Into viviparous oa’ oviparous ; which places the dividing 
line between Mammals, and the inferior Vertebrates. 
2. Into warm-blooded and cold-blooded, or those having perfect, 
and those having imperfect, circulation; which places the line 
between Mammals and Birds, on one side, and Reptiles and 
Fishes, on the other. 
3. Into pulmonate and branchial, or those with lungs, and 
those with gills; which places the line between Mammals, Birds 
and Reptiles, on one side, and Fishes, on the other. 
Now the characteristic of the first of these methods of sub- 
division is that on which the hemitypic group of the first class, 
or that of Mammals, is based. The characteristic of the third 
is that on which the hemitypic group of the third class, or the 
Reptilian, is based. Hence, the characteristic of the second 
should be, if the analogy holds, that on which the hemitypie 
group of the second class, or that of Birds, rests for its most 
fundamental distinction. 
3. Geological history.—It has been observed, on page 318, that 
the Vertebrate subkingdom has well-drawn limits below, instead — 
_of tapering downward into Mollusks or Articulates. This fea 
ture of the subkingdom is further evident from the fact in geo- 
logical history that the earliest species of Fishes were not of 
the lower group, that of Teliosts, but of the two higher, or those 
of Ganoids and Selachians. The Vertebrate type did not orig! 
nate therefore in the subkingdom of Mollusks, or of Articulates; 
neither did it start from what might be considered as its base, 
that is, the lower limit of the class of Fishes; but in intermedi- 
ate types, occupying a point between typical Fishes and the 
classes above. : 
Moreover, the inferior group did not come into existence until ‘ 
the Cretaceous period, in the latter part of geological history, 
when the Reptilian age was commencing its decline. od 
In the Devonian age, or closing Silurian, appeared the first 
Ganoids and Selachians. In the Carboniferous, Reptiles were 
introduced,—first the inferior Amphibians, and then typical ” 
ies. Afterward, in the early part of the Reptilian age, 9 
eptilian life was in course of expansion, there were the first of 7 
the Reptilian Birds and the first of the Marsupials or pees me. 
Mammals (with probably some typical species of each of these 
lasses). Thus — Vertebrate type, commencing at the point — 
