254 AMERICAN TESTUDINATA. Part II. 



locomotive organ of which is still the vertebral column with the movable ribs, as in 

 the Snakes. In the Dibamus of New Guinea, there appear, for the first time, visible 

 extremities, small, slender, toeless, scaly hind feet. In the Bipes of New Holland 

 these become somewhat larger, and in Brachymeles, rudiments of anterior extremities 

 are added. In the genus Evesia, these extremities are still undivided ; in the Brach- 

 ymeles projaer they have, in front and behind, two toes; in the South-Euro- 

 pean Seps, we find already three toes in front and behind ; in the Scincus five,^ 

 in front and behind, but the fore feet are still weak and do not yet carry the 

 body so swiftly and easily over the earth as those of the Lizards, but these also, 

 with their perfectly developed feet, are still assisted by the motion of the vertebral 

 column. From this point of the series up to the Turtle, there is a great stride, 

 for in them the head and neck are free, much freer than in any of the Saurians 

 whatsoever. The vertebral column has become stiff; the four feet are the only 

 locomotive organs; and yet in the marine Turtles, the fore feet exceed greatly in 

 power the hind pair, and it is only in the land Turtles that we find at last all the 

 four feet perfectly equal in sti'ength, affording four j^rops or supports, upon which 

 the whole body moves slowly foi'wai'd, like a house on rollers. 



This is the natural series of the orders in the class of true Reptiles. Let us 

 now consider the class of Amphibians from the same point of view. The Cfscilia, 

 the lowest Batrachian, is a long-drawn, serpentrlike animal, moving by means of 

 undulations of the vertebral column. In one of our southern Ichthyoid Batrachians, 

 called Siren, there arise two feeble feet in front, or rather a pair of diminutive 

 anterior limbs project from behind the gills. In the German Proteus, or the North 

 American Amphiuma, four legs are already perceptible, having from two to three 

 toes, and the Salamanders, which at present extend over the whole surface of the 

 globe, walk, like the Lizards, on four well developed feet, using like them, however, 

 the whole dorsal column as a locomotive organ. From these, again, we have a 

 stride up to the Frog. The spine has become stiff; all lateral motion has ceased 

 in it, and, as in the Reptile when in its highest development, so with the most 

 perfect Batrachian, the four feet are the only locomotive organs. This is the 

 series of the Amphibians.^ 



If we now compare the highest Reptile, the Turtle, with the highest Amphi- 

 bian, the Frog, the locomotive organs in both being completely developed, and the 

 spine serving no longer for locomotion, we find the latter ready to be applied to 

 other purjioses. A step towards this is made in the Frog. The caudal bone is 

 separated sharply and distinctly from the rest of the spine, as is also the neck, 



' For further details respecting the series of the ^ Compare the ilhistrations of this series in my 



family of Scincoids, see Part I., Chap. 1, Sect. 12. Lectures on Comparative Embryology, p. 8 to 10. 



