XVII] THE COMPARISON OF RELATED FORMS 1073 



of the skull, conspicuous as they are at first sight, will be found 

 easy to bring under the conception of a simple and homogeneous 

 transformation, such as would result from the application of some not 

 very comphcated stress. For instance, the corresponding coordinates 

 of Aceratherium tridactylum, as shewn in Fig. 539, indicate that the 



Fig, 537, The pelvis of Archaeopteryx and of Apatornis, with three 

 transitional types interpolated between them. 



essential difference between this skull and the former one may be 

 summed up by saying that the long axis of the skull of Aceratherium 

 has undergone a slight double curvature, while the upper parts of 

 the skull have at the same time been subject to a vertical expansion, 

 or to growth in somewhat greater proportion than the lower parts. 

 Precisely the same changes, on a somewhat greater scale, give us 

 the skull of an existing rhinoceros. 



