1072 THE THEORY OF TRANSFORMATIONS [ch. 



in Fig. 537 the complete series is represented, beginning witli the 

 known pelvis of Archaeopteryx, and leading up by our three inter- 

 mediate hj^othetical types to the known pelvis of Apatornis. 



Among mammalian skulls I will take two illustrations only, one 

 drawn from a comparison of the human skull with that of the 

 higher apes, and another from the group of Perissodactyle Ungulates, 

 the group which includes the rhinoceros, the tapir, and the horse. 



Fig. 536. The second and third intermediate coordinate networks, 

 with their corresponding inscribed pelves. 



Let US begin by choosing as our type the skull of Hyrachyus 

 agrarius Cope, from the Middle Eocene of North America, as figured 

 by Osborn in his Monograph of the Extinct Rhinoceroses* (Fig. 538). 



The many other forms of primitive rhinoceros described in 

 the monograph differ from Hyrachyus in various details — in the 

 characters of the teeth, sometimes in the number of the toes, and 

 so forth; and they also differ very considerably in the general 

 appearance of the skull. But these differences in the conformation 

 * Mem. Amer. Mies, of Nat. Hist, i, in, 1898. 



