236 AMERICAN TESTUDINATA. Part II. 



evidence, though they were imdoubtedly led to their results by that instinctive 

 appreciation of the true relations among organized beings, which, in the history of 

 science, is so often found to precede the practical demonstration and establishment 

 of final results. Certainly, it is an unquestionable fact, that correct views are 

 frequently propounded upon suljjects of natural science, the proof of which, in the 

 first imperfect state of our knowledge, is still wanting. In the case before us, we 

 shall see how the practice of naturalists has generally led them to results which 

 have not been, till now, susceptible of demonstration; but I hold that the possi- 

 bility of thus accounting in the end for views instinctively adopted, and so often 

 generally accepted, is in itself satisfactory evidence that the principles which fur- 

 nish the final demonstration are true to nature. 



It might seem superfluous here to show that the class of Reptiles belongs 

 to the type of Vertebrates, did it not afford an ojaportunity of showing that the 

 definition of the great branches of the animal kingdom given above is correct. 

 It has been stated ^ that these primary divisions did not rest upon pecuHar struc- 

 tures, upon a distinct combination of given systems of organs, but exclusively upon 

 a plan of structure. To show that Reptiles are Vertebrates, it may be sufficient, 

 in jiractice, to exhibit their solid internal frame; but that this cannot be considered 

 as the essential characteristic of a vertebrated animal is amply proved by the 

 fact that Amphioxus no more has a skeleton, properly speaking, than the Myxi- 

 noids and Petromyzontes ; yet no one doubts that their true position is among 

 Vertebrates. Again, in Testudiuata, the largest part of the skeleton is truly exter- 

 nal, their bony box being only covered by comparatively thin scales or a naked 

 skin. There is, indeed, no class in which a greater diversity of structure is exhib- 

 ited than among Reptiles ; for, without mentioning the Batrachians, which constitute 

 a class by themselves, what extraordinary difference is there not between Snakes, 

 Lizards, and Turtles ! To show that notwithstanding this variety of structure, these 

 animals actually belong to the branch of Vertebrata, is the object I have in 

 view ; and if it can be shown that so diversified a class belongs to that type, accord- 

 ing to our understanding of the term branch, we shall have the required proof 

 that our definition is true to nature. Now I have stated that branches are founded 

 upon different plans of structure. What is, then, that plan in Vertebrates which 

 unites Amphioxus, Cyclostomes, Sharks, Skates, Bony Fishes, Ichthyoids, Salamanders, 

 Toads, Frogs, Snakes, Lizards, Crocodiles, Turtles, Birds, Whales, Marsupials, our com- 

 mon Quadrupeds, Bats, Monkeys, and Man, which includes them all in one and the 

 same group, and shows that group to be natural ? 



The body of all Vertebrates represents a double tube, one above the other, 

 separated by a longitudinal axis, and varying in amplitude and in fonu at dif- 



1 See Part I., Chap. 2, Sect. 1, p. 141-144. 



