6 GENERAL HISTORY OF BIRDS. 



These characters could be indefinitely supplemented by adding such as 

 would serve to contrast with the Mammals, the Amphibians, the Fishes, and 

 other classes of vertebrates, but for our present uses those given suffice for all 

 practical purposes. By themselves, however, most of them are of compara- 

 tively little taxonomic value. Some of the Dinosaurian reptiles had hind feet 

 nearly like those of birds and triassic representatives of the order left, on the 

 shores that became the red sandstone of the Connecticut valley, numerous im- 

 prints that were long supposed to be those of the feet of birds; the transfor- 

 mation into wings of the fore-limbs is no greater a deviation from a normal 

 limb than is the one, equally adapted to flying, of a Pterodactyl reptile or that, 

 modified as a flipper for swimming, of an Ichthyosaurian reptile; the feathery 

 covering of a bird is not more remarkable or a more extreme modification of 

 the integumental investiture than is the shell of a chelonian reptile (tortoise); 

 most tortoises, too, have horny sheaths capping the jaw bones; the variations 

 in the structure of the heart and the circulatory apparatus of reptiles are so 

 great that little strain on taxonomy would be exercised in admitting the avian 

 modification within the limits of the class, and it is quite possible that Ptero- 

 dactyls may have had a complete circulation and warm blood and not impossi- 

 ble even that some at least of the Dinosaurians had; the Crocodilians even of 

 the present day approach such a structure. 



It is therefore, we repeat, not by any one character but the aggregate of 

 characters that the group of birds is separated from the reptiles and that group 

 is so natural and so well represented by numbers that convenience is subserved 

 by its difl^erentiation as a class. Furthermore, the principal forms of each 

 class are antitypical to each other. The cold-blooded and creeping reptile is 

 generally regarded as the very opposite of the quick-blooded and soaring bird. 

 In the present age of the earth the two contrast rather than compare and so 

 we represent them in our classification. 



The Birds contrast with the reptiles not only in their physiological char- 

 acteristics but in their homogeneity. That homogeneity is indeed wonderful 

 and the class thereby contrasts not only with the reptiles but with every class 

 of vertebrates. Allusion has been made to the variation among the reptiles ; in 

 the class of mammals there are forms adapted for a flight as vigorous as that 

 of most birds (Bats) and others which swim in the seas as well as fishes 

 (Cetaceans) Among the fishes we find even still greater variation; there are, 

 for instance, forms higher than long and there are others more slender than 

 a whip-lash. The Amphibians, the Selachians and the Marsipobranchs mani- 

 fest little less if any less morphological variation than those specified. In con- 

 trast with such, the uniformity existing within the class of Birds is great 

 indeed. What is true of one, in general terms, is true of all. The species, 

 numerous as they are, differ chieffy in matter of detail. 



