Chap. I. RANK OF THE TESTUDINATA. 237 



ferent points of their longitudinal diiuneter. These tubes are surrounded by walls, 

 varying in thickness, as the spaces they inclose vary in size, the upper one con- 

 taining the centres of the nervous system, the lower one the organs through which 

 life is maintained ; while the walls, in connection with the intervening longitudinal 

 axis, constitute a locomotive apparatus, and serve also to sustain the relations with 

 the surrounding media. 



These characteristics of the type of Vertebrates do not necessarily imply a definite 

 structure ; they »pply i^s well to the imperfectly organized Amphioxus as to Man, for 

 they do not involve the idea of a distinct head, nor that of locomotive appendages 

 arranged in pairs, nor that of a branchial or pulmonary system of respiration, nor that 

 of a heaa"t as the centoe of circulation, nor indeed any of those anatomical and histo- 

 logical differences or pecuharities which are constantly and, in my opinion, errone- 

 ously introduced in the characteristics of the great types of the animal kingdom. 

 The external development of the skeleton of the Turtle no longer seems an 

 anomaly, when we remember that it forms a part of those walls which surround 

 the spinal cavity on the one hand, and the abdominal cavity on the other. 



If we next consider the Reptiles as a class, we must remember that ever since 

 Linnfeus these animals have been considered as one class. Cuvier, and with him 

 all herpetologists, have agreed in considering them all as one class. We find 

 de Blainville, for the first time, insisting upon the separation of the Batrachians 

 from the other Reptiles as a distinct class. This view has also been adopted by 

 Milne-Edwards, while Wagler has separated a few of their extinct types, the 

 Ichthyosauri, the Plesiosauri, and the Pterodactj-li, to miite them with the Orni- 

 thorhynchus and Echidna as one class, under the name of Grjphi. The incon- 

 gruity of this combination is so obvious, now that these fossil animals have been 

 described in such a masterly manner by R. OAven, that I will not dwell upon 

 its artificial character here. But the separation of the Batrachians from the other 

 Reptiles as a class deserves a .special notice, and if tlie definition I have given 

 above of a class, as such, is correct, the result cannot be doubtful. I have stated 

 that a class was defined by the manner in which the plan of structure of the 

 branch to v.hich it belongs is carried out. I have condensed that definition l)y 

 saying, that the limitation of a class is a question of ways and means. Now, 

 before applying this definition to the question of the separation of Batrachians 

 from other Reptiles, I would make two remarks; In the first place, that this 

 definition was not made to suit the case, but was arrived at by a critical con- 

 sideration of the foundation upon which tliose cla.sses rest, alx)ut whose natural 

 limits there have never existed great doubts among naturalist.s, such as the 

 class of Mammalia, that of Bird,s, that of Cephalopods, that of Gasteropods, that 

 of Insects, that of Crustacea, and that of Echinoderms; in the second place, that 



