78 On Farallel Belations of the Classes of Vertebrates, 



Keptiles, and the other towards tlie three higher classes of Verte- 

 brates collectively, but especially Mammals and Birds. 



It is plain from the preceding that the sub-kingdom of Verte- 

 brates, instead of tailing off into the Invertebrates, has well pro- 

 nounced limits below, and is complete within itself. 



2. Distinctive Features of the Reptilian Division of Birds. — The 

 skeleton of the fossil Bird, discovered at Solenhofen, has some 

 decided Reptilian peculiarities, as pointed out by Wagner, Owen, 

 and others. But even if perfect, it could n^ot indicate all the 

 Keptilian features present in the living animal. It is therefore 

 a question of interest, whether the relations of the hemitypic to 

 the typical species in the two classes Mammals and Eeptiles — 

 one superior to that of Birds, and the other inferior — afford any 

 basis for conclusions with regard to characteristics of the hemi- 

 typic Birds undiscoverable by direct observation. The following 

 considerations, suggested by analogies from the classes just men- 

 tioned, may be regarded as leading to unsatisfactory results ; and 

 yet they deserve attention. 



A. Mammals. — (1.) It is a fact to be observed that the hemi- 

 typic Mammals are as truly and thoroughly Mammalian, as 

 regards the fundamental characteristic of the type — the suckling 

 of their young — as the typical species. 



(2.) The departure from the typical Mammals is small in the 

 adult individuals, especially the adult males. But it is profoundly 

 marked in their young, they thus approximating in period of birth 

 and some other respects to oviparous Vertebrates. 



B. Reptiles. — (1.) The adult Amphibians, or hemitypic Reptiles, 

 depart but little from the typical Reptiles, either in structure or 

 habits. 



But (2.) the young, in their successive stages, from the egg 

 upward, partake strikingly of characters of the inferior class of 

 Fishes. 



The law seems, then, to be, that the species of the hemitypic 

 group have their principal or most fundamental resemblance to 

 those of the class or classes below in the young state. We should 

 hence conclude that the young of the Reptilian Birds or Erpetoids 

 possessed more decided Reptilian peculiarities than the adults. — 

 What these unknown peculiarities, if real, were, we can infer 

 only doubtingly from the analogies of the known cases already 

 considered. 



The characteristic of the intermediate type, on which the in- 

 termediate character depends, is, in the case of both Mammals 

 and Reptiles, that particular one which is the special distinction 

 of the inferior type. The types inferior to Mammals are oviparous^ 

 and hence the hemitypic Mammals are semi-oviparous. The type 



