32 HISTORY OF THE HUMAN BODY 



the birds, the other to the mammals. Of the first of these, the 

 oldest group is that of the Rhynchocephalia, mainly fossils, 

 but with a single living species, which fate has preserved in 

 New Zealand, the Sphenodon (Hatteria). This represents 

 the ancestor of lizards and snakes, Lacertilia and Ophidia re- 

 spectively, and also a group of extinct reptilian giants, the 

 dinosaurs, whose nearest living allies are the crocodiles. Here 

 this line would have ended, so far as human knowledge is 

 concerned, had it not been for the chance discovery, about 

 the middle of the nineteenth century, of two specimens of one 

 of the most remarkable " missing links " ever found, the 

 Archceopteryx, a form midway between reptiles and birds, and 

 of undoubted affinity to the stem of the dinosaurs. This 

 creature was bird-like, possessed wings and a certain number 

 of contour feathers, but had a long vertebrated tail, several 

 free digits in the hand, furnished with curving claws, and a 

 heavy jaw containing conical teeth, reptilian in character. This 

 discovery, followed by that of the toothed birds, completed 

 the chain of evidence, and supplied one of the most isolated 

 groups of vertebrates with a definite line of ancestry. 



The other line of reptiles, which may have arisen from the 

 Stegocephali more or less independently of the first, was that 

 beginning with the theromorphs, an extinct group, many of 

 which attained a gigantic size. Some members of this group 

 are so near the mammals in many particulars that it has been 

 only with the greatest care, and through the consideration of 

 all the available parts, that their reptilian nature has been de- 

 termined. In studying the remains of these forms, especially 

 those of the sub-group of theriodonts, the most of which were 

 small animals, like the earliest mammals, it seems impossible 

 not to assign them a close relationship to the latter, probably 

 that of actual ancestry. Indeed, there is at present but one 

 other claimant for that position, and that is the group of 

 Stegocephali, and as these were contemporary with the 

 theromorphs, and at one time probably graded into them 

 by imperceptible transitions, the two views are not very wide 

 apart. All things considered, it seems that the gap between 



