XVII] THE COMPARISON OF RELATED FORMS 1065 



near to the origin of the system, the whole region of the head, the 

 opercular orifice and the pectoral fin, practically unchanged in form, 

 size and position; and it shews a greater and greater apparent 

 modification of size and form as we pass from the origin towards 

 the periphery of the system. 



In a word, it is sufficient to account for the new and striking 

 contour iri all its essential details, of rounded body, exaggerated 

 dorsal and ventral fins, and truncated tail. In like manner, and 

 using precisely the same coordinate networks, it appears to me 

 possible to shew the relations, almost bone for bone, of the skeletons 

 of the two fishes; in other words, to reconstruct the skeleton of 

 the one from our knowledge of the skeleton of the other, under 

 the guidance of the same correspondence as is indicated in their 

 external configuration. 



The family of the crocodiles has had a special interest for the 

 evolutionist ever since Huxley pointed out that, in a degree only 

 second to the horse and its ancestors, it furnishes us with a close 

 and almost unbroken series of transitional forms, running down 

 in continuous succession from one geological formation to another. 

 I should be inclined to transpose this general statement into other 

 terms, and to say that the Crocodilia constitute a case in which, 

 with unusually little complication from the presence of independent 

 variants, the trend of one particular mode of transformation is 

 visibly manifested. If we exclude meanwhile from our comparison 

 a few of the oldest of the crocodiles, such as Belodon, which differ 

 more fundamentally from the rest, w^e shall find a long series of 

 genera in which we can refer not only the changing contours of the 

 skull, but even the shape and size of the many constituent bones 

 and their intervening spaces or "vacuities," to one and the same 

 simple system of transformed coordinates. The manner in which 

 the skulls of various Crocodilians differ from one another may be 

 sufficiently illustrated by three or four examples. 



Let us take one of the typical modern crocodiles as our standard 

 of form, e.g. C. porosus, and inscribe it, as in Fig. 527, a, in the 

 usual Cartesian coordinates. By deforming the rectangular network 

 into a triangular system, with the apex of the triangle a little way 

 in front of the snout, as in b, we pass to such a form as C. americanus. 



