238 AMERICAN TESTUDINATA. Part II. 



it is entirely erroneous to consider, as is universally clone, that the classes exhibit 

 modifications of the plan of structure of their respective branches. 



It is no more true that Fishes, Eeptiles, Birds, and Mammalia exhibit respectively 

 modifications of the plan of structure of Vertebrates, than that Insects, or Crus- 

 tacea, or Worms are respectively modifications of the type of Articulates, or the 

 different classes of Mollusks and Eadiates, modifications of their respective types. 

 A Fish is as truly a Vertebrate as any Bii-d or Mammal ; the plan is not at 

 all modified ; it is only executed in different ways and with different means. 

 The plan which characterizes Vertebrates is no more modified in the Fish than 

 in the Eeptile ; the plan of Articulates is no more modified in Insects than in 

 Crustacea or Worms; the plan of Mollusks, as a plan, is the same in Cephalopods 

 as in Gasteropods and Acephala ; that of Eadiates, the same in Polyps as in 

 Acalephs and Echinoderms. What, then, constitutes the difference of each class 

 in the same branch ? It is the manner in which the plan of the branch to which 

 they respectively belong is carried out. They are respectively characterized by 

 the way in which, and the means with which, they are built up. The idea of 

 radiation which is inherent in the plan of structure of Eadiates is the same in aU 

 Eadiates, in Polyps as well as in Medusae and Echinoderms ; but in the Polyps it 

 is expressed in one way, in the Acalephs in another, and in Echinoderms in still 

 another. This is equally true of all the other classes, with reference to the plan 

 of their respective branches. The different ways in which, and the different means 

 with which each plan is executed in its respective classes, go far to show that 

 the branches themselves are founded in nature, for the means employed in carrying 

 out these different plans in a variety of ways, in their different classes, are every- 

 where homological, and homological only within the limits of the same branch. 

 We can trace no true homology between the systems of organs in Vertebrates 

 and those in Articulates, nor between these and those of MoUusks ; and a critical 

 examination shows that the structure of Eadiates is not homological with that of 

 Mollusks. 



Truly homological systems of organs, then, more or less complicated, constitute 

 class characters ; l)ut, again, these homologies are only general as far as the branch 

 is concerned, while within each class special homologies only can be traced. Had 

 these distinctions been made before, what an amount of confused discussion might 

 have been spared respecting homologies in the animal kingdom ! I trust this state- 

 ment, the correctness of which may easily be tested by a comparison of the 

 Batrachians and the true Eeptiles, will put an end to the useless and puerile 

 attempts to homologize every point of ossification in any class of the Vertebrates 

 with some part or other of the skeleton of all the other members of that type. 

 I hope also it may prevent such fanciful investigations from being extended into 

 the study of the other systems of organs. 



