296 



HISTORICAL GEOLOGY. 



it is observed that in many the paired fins are curiously 

 formed they are a sort of limbs fringed with fin. Xow, 

 a large number of Devonian fishes (Fig. 212) have this 

 style of paired fins. In the third place, all the living 



FIG. 217. 



FIG. 218. 



FIGS. 217, 218. Nearest living allies of Devonian fishes : 217. Polypterus. 218. Ces- 

 tracion Phillippi (a living cestraciont from Australia). 



Ganoids given above (Figs. 215, 217) have a more or less 

 perfect lung, and supplement their water-breathing with 

 air-breathing, in the manner of some amphibian reptiles. 

 It is almost certain that the same was true of Devonian 

 Ganoids. And yet, with all these reptilian characters, 

 all Devonian fishes had cartilaginous skeletons like the 

 embryos of Teleosts. 



We wish now to take advantage of these facts to an- 

 nounce a very general law. The first introduced exam- 

 ples of any family, order, or class, are not typical forms 

 of that family, order, or class, but intermediate forms or 

 connecting links With other families, orders, etc. From 

 such intermediate forms or connecting links have been 



