The Peinciples of Palaeontology. 147 



ber of palaeontologists of the present day are other wise engrossed. 

 When they possess a sufficient quantity of materials, they study 

 minutely the variations exhibited by each separate form, and es- 

 tablish "series of forms," keeping in view both variations and 

 mutations. The analysis is carried farther than it formerly was^ 

 but the synthetic conclusions are v^hat complete it successfully. 

 The denominations and divisions may afterward undergo some 

 change, according to the- preferences of authors, but the import- 

 ant facts remain, established, and the evolution of the group with 

 all its important details is understood. 



These delicate researches recorded in vrorks of difficult read- 

 ing, but whose conclusions are of the greatest interest, have been 

 carried on thus far principally in the class of MoUusks. We 

 shall farther on point out the significance of the recent work of 

 Hyatt on an important family of Ammonites. Buckman has- 

 gradually brought into notice the Ammonites of the Bajocian stud- 

 ied from the same point of view. Analyses of a similar nature 

 have had an especial bearing on the following groups : Among 

 the Mollusks, the Cancellarias (Hoernes), the Inocerami, Halobiay. 

 the Unionidfe of the Slavonic deposits (Penecke), a very great 

 number of Brachiopods (Davidson, (Ehlert) ; among the sea- 

 urchins, the genus Ananchytes; the plants considered as the pro- 

 genitors of living forms (de Saporta). These researches are more 

 fertile in results than those which confine themselves to distin- 

 guishing more than a hundred species of- Unio in French waters, 

 or to creating unconsciously several species out of two bracches 

 of the same plant. 



What conclusion is to be drawn then from all that has been 

 said on the subject of the limitation of species in Palaeontology '^ 

 At the present hour every criterion is at fault. The limitation 

 of species is, as has often been remarked, a matter of apprecia- 

 tion. We group under one denomination the most closely related 

 forms, those which are united by many degrees of transition, but 

 all separable from different forms by an appreciable interval. 

 Frequently the transitions are defectivebetween the forms which 

 are found in different layers of the same locality, whilst most 

 horizons are characterized by especial species for every group of 

 fossils. We shall presently see the cause of this phenomenon,, 

 which everywhere presents numerous exceptions. 



