XVII] THE COMPARISON OF RELATED FORMS 759 



Cartesian co-ordinates (Fig. 388) ; while the corresponding co- 

 ordinates of the very different pelvis of Apatornis are represented 

 in Fig. 389. In Fig. 390 the outlines of these two co-ordinate 

 systems are superposed upon one another, and those of three 

 intermediate and equidistant co-ordinate systems are interpolated 

 between them. From each of these latter systems, so determined 

 by direct interpolation, a complete co-ordinate diagram is drawn, 

 and the corresponding outline of a pelvis is found from each of 



Fig. 392. 



The second and third intermediate co-ordinate networks, 

 with their corresponding inscribed pelves. 



these systems of co-ordinates, as in Figs. 391, 392. Finally, in 

 Fig. 393 the complete series is represented, beginning with the 

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

 mediate hypothetical 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 



