THE EXTINCTION OF SPECIES AND OF GROUPS 239 



Trilobites which became extinct at the end of 

 Primary times no longer comprises after the Car- 

 boniferous epoch more than one branch, that of 

 the Phillipsice, which continues as far as the Per- 

 mian with nothing but insignificant mutations or 

 variations. .The Brachiopods of the family of the 

 Spiriferidse, so brilliantly represented in Primary 

 times at the end of their existence in the Lias, are 

 no longer represented except by two small genera, 

 Spiriferina and Suessia, with forms very little 

 varied. The tetrabranchial Cephalopods, whose 

 forms, varied to infinity, were the ornament of the 

 Silurian seas, already lose the major part of their 

 branches in the Devonian, and after the end of 

 the Trias only figure in the shape of nautilo'id shells 

 of so uniform a type that palaeontologists have 

 some difficulty in distinguishing species among 

 them. In the Vertebrates the phenomenon is 

 likewise very frequent. The evolution of the 

 branch of the Dinotheria, for instance, passes, in 

 Europe, through the whole of the Miocene times 

 without any other variation than a regular pro- 

 gression in size, so that, without this characteristic, 

 all specific distinction would be impossible. Many 

 similar remarks might be made regarding other 

 groups extinct or in course of extinction ; for ex- 

 ample, in the Palseotheridse, the Tapiridae, the 

 Oreodontidae, the Anoplotheridae, the Mastodons, 

 the Taxodons, the Hyracoi'ds, the Hyaenodontidae, 

 etc. It must, however, be acknowledged that the 

 law of Rosa constitutes in one way a vicious circle, 

 for it would be quite as easy to assume that if 

 branches which have arrived at the termination of 



