76 On Parallel Relations of the Classes of Vertebrates, 



received, each, excepting the lowest, consists of, first a grand 

 typical division, embracing tlie majority of its species, and secondly, 

 an inferior or hemitypic division, intermediate between the typical 

 and the class or classes below. 



Before proceeding with our illustrations of this point, a word 

 may be added in behalf of these four classes. In order to appre- 

 ciate their true value, it is necessary to have in vicAv the type-idea 

 which is the basis of the fundamental characteristics of each, and 

 which is connected with the existence of three distinct habitats for 

 life — the water, the air, and the land : thai in Fishes, this idea 

 is that of swimming aquatic life ; in Reptiles, that of creeping 

 terrestrial life ; in Birds, that of flying aericd life ; in Mammals, 

 that of terrestrial life, again, but in connection with a higher grade 

 of structure, the Mammalian. The type-idea is expressed in the 

 adults both of the typical and hemitypic groups; and any attempt 

 to elevate the hemitypic into a separate class tends to obscure 

 these ideal relations of the groups in the natural system of Verte- 

 brates. 



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, ovipai^ous. The 

 species of the inferior or hemitypic group of Mammals, partake, 

 therefore, in some degree, of an oviparous nature, as the term semi- 

 oviparous or Odtocoid implies. 



In fact, all Vertebrates, excepting Mammals, are typically ovi- 

 parous, although some cases of viviparous birth occur among both 

 Keptiles and Fishes. In the viviparous Mammals, the embryo, 

 during its development, derives nutriment directly from the body 

 of the parent until birth, and also for a time after birth ; while 

 in the viviparous Fish, the Selachians excepted, there is simply a 

 development of the egg internally, in the same manner essentially 

 as when it takes place externally. Applying, then, the term ovi- 

 parous to all cases in which the embryo is shut off from any kind 

 of placental nutrition, Ileptiles and Fishes, with the exception 

 mentioned, are as essentially oviparous as Birds. Hence, the 

 Obtocoids or non-typical Mammals are actually intermediate in 

 this respect, and in others also, between the typical Mammals, on 

 one side, and the inferior oviparous Vertebrates collectively, on 

 the other. 



(2.) Again, the class next below Birds is that of Reptiles. 

 And, correspondingly, the inferior or hemitypic group of Birds is 

 Beptilian in some points of structure. 



(3.) Again, the class next below Reptiles is that of Fishes ; 

 and therefore the inferior or hemitypic group of Reptiles is the 

 intermediate or Ichthyoid one of Amphibians — the young of frogs 



