An Introduction to Zoology. 39 



There prevails a remarkable unity of design throughout the whole series 

 of the vertebrated animals, including fishes, reptiles, birds, and mammalia ; 

 in all we recognise an identity of general structure in the skull and verte- 

 bral column ; in all, except the apodal serpents and fishes, we recognise a 

 similar conformation in the locomotive extremities, bone for bone admitting 

 of being pointed out, though developed in very different proportions. 



Thus the same elements are, by a wonderful adjustment of modifications, 

 made to constitute a fin in the whale, a paddle in the turtle ; to support a 

 membranous wing in the bat, and a feathery one in the bird ; in beasts of 

 prey, to afibrd the organs of rapid pursuit, of tenacious grasp, and of power 

 to rend in pieces by the sharp retractile claws with which it is armed ; 

 and in man, to provide that wonderful instrument, efiective of all other 

 instruments, the human hand, co-ordinate with human reason. In every 

 case we see, indeed, a necessary harmony existing through the several 

 parts of the animal frame : a carnivorous stomach evidently requires organs 

 very different to sieze its appropriate prey, from the cloven hoof of the 

 innocent graminivorous ruminantia. And not the extremities only, but many 

 other portions of the skeleton must be modified so as to afford adequate 

 attachments to the muscles destined to move those extremities ; and the 

 teeth have evidently the same relations. Hence what are called the laws 

 of the co-existence and co-relation of the several parts of the animal 

 frame, which are so very perfect, that from a single fossil fragment, the 

 mere joint of a toe, perhaps, we are able boldly to pronounce as to the class 

 and order to which the original has belonged. 



In fishes, the pectoral and ventral fins appear to represent the four ex- 

 tremities of quadrupeds, but are very difi"erently organised ; and the support 

 of the gills require a peculiar set of osseous arches, and they are covered 

 by another peculiar series of flat opercular bones : but to represent these, 

 as Geoff"roy St. Hilaire has done, as developments of the internal little 

 ossicles of the ear of mammalia, is to violate every analogy of form, relation, 

 or function, and to substitute arbitrary canons, which may make any thing 

 out of any thing, and can only lead to conclusions of the most gratuitous 

 absurdity. 



The extremities are partially or wholly dispensed with. Partially in 

 the whale tribe, which, being mammalia destined to the life of fish, do not 

 require and are therefore not furnished with the hinder feet : — wholly 

 in serpents, whose ribs are so constructed as to enable them to crawl on 

 their extremities. In some of these, however, rudimental traces of the 

 legs and feet may be seen ; in others they entirely vanish. 



As we have seen the ribs employed as instruments of progression, so in 

 the draco volans we find them expanded on cither side, so as to support 

 two flat lateral flaps, which almost act as substitutes for wings, enabling 

 the animal to take long flying leaps. 



We cannot look at this great variety of effects, produced by slight modi- 



