" DIE STAMME DES THIERREICHS " 67 



which often separate the forms of fossil animals 

 from one another and constitute at times an in- 

 superable obstacle to the perception of the relations 

 of continuity which must have linked these forms 

 together, there can, fortunately, be chosen, as we 

 have seen above for living forms, a few special groups 

 which lend themselves better than others to the re- 

 construction of continuous series, or series of forms 

 (Formenreihe), according to Neumayr's expression. 

 To obtain this result we must deal with a very large 

 number of individual subjects gathered from a 

 series of strata in regular succession without gaps 

 and in analogous conditions of deposit. The first 

 palaeontologists to adopt this course were, on the one 

 hand, Hilgendorf, in his memorable work on the 

 variations of the Planorbis multiformis of the 

 fresh- water beds of Steinheim ; on the other, 

 Waagen, in his researches on the series of the 

 Ammonites of the group Ammonites subradiatus. 

 The number of series of forms which it has till now 

 been possible to reconstitute is more limited than 

 one might think on a priori grounds. However, 

 for Primary times, certain groups of Brachiopods, 

 of Polyps, and of Crinoids show us series of gradual 

 variations. In Secondary times there may be 

 quoted among the best examples the shells of 

 Ammonites, a few kinds of Lamellibranchs, Phola- 

 domyse. Inoceramiaa, Halobise, Brachiopods, and also 

 a few kinds of Urchins. But it is the Tertiary epoch 

 which best lends itself to this kind of research, 

 thanks to the abundance of fossils and to their 

 excellent state of preservation. A long time before 

 Darwin's book, Moritz Homes had shown perfectly, 



