80 On Pamllel Relations of the Classes of Vertehrates, 



the young Bird during an early stage in its development. Tliis 

 peculiarity in the vascular system of the young Bird of the pre- 

 sent day ceases with the beginning of respiration. But in the 

 Reptilian birds it may have continued 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 subdividing 

 Vertebrates, and the connection therewith of the characteristics 

 of the hemitypic groups. These three methods are — 



1. Into viviparous and 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 Beptiles and Fishes, 

 on the other. 



3. Into pidmonate and branchial, or those with lungs, and those 

 with gills ; which places the line between Mammals, Birds, and 

 Beptiles, on one side, and Fishes on the other. 



Now the characteristic of the frst 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 hemitypic group of 

 the second class, or that of Birds, rests for its most fundamental 

 distinction. 



3. Geological History. — It has been observed, on page 78, that 

 the Vertebrate sub-kingdom has well-drawn limits below, instead 

 of tapering downward into Mollusks or Articulates. This feature 

 of the sub-kingdom is further evident from the fact in geological 

 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 originate, therefore, 

 in the sub-kingdom 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 intermediate 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. 



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 

 species. Afterward, in the early part of the Reptilian age, as 

 Reptilian life was in course of expansion, there were the first of 



