Fossil Mammals 189 



wished, different observers will put different interpretations upon 

 it, as in the notorious case of the Steinheim shells 1 . The ludicrous 

 discrepancies which often appear between the phylogenetic "trees" 

 of various writers have cast an undue discredit upon the science and 

 have led many zoologists to ignore palaeontology altogether as un- 

 worthy of serious attention. One principal cause of these discrepant 

 and often contradictory results is our ignorance concerning the exact 

 modes of developmental change. What one writer postulates as 

 almost axiomatic, another will reject as impossible and absurd. Few 

 will be found to agree as to how far a given resemblance is offset by 

 a given unlikeness, and so long as the question is one of weighing 

 evidence and balancing probabilities, complete harmony is not to 

 be looked for. These formidable difficulties confront us even in 

 attempting to work out from abundant material a brief chapter 

 in the phylogenetic history of some small and clearly limited group, 

 and they become disproportionately greater, when we extend our 

 view over vast periods of time and undertake to determine the 

 mutual relationships of classes and types. If the evidence were 

 complete and available, we should hardly be able to unravel its 

 infinite complexity, or to find a clue through the mazes of the 

 labyrinth. " Our ideas of the course of descent must of necessity be 

 diagrammatic 2 ." 



Some of the most complete and convincing examples of descent 

 with modification are to be found among the mammals, and nowhere 

 more abundantly than in North America, where the series of con- 

 tinental formations, running through the whole Tertiary period, is 

 remarkably full. Most of these formations contain a marvellous 

 wealth of mammalian remains and in an unusual state of preserva- 

 tion. The oldest Eocene (Paleocene) has yielded a mammalian fauna 

 which is still of prevailingly Mesozoic character, and contains but 

 few forms which can be regarded as ancestral to those of later times. 

 The succeeding fauna of the lower Eocene proper (Wasatch stage) 

 is radically different and, while a few forms continue over from the 

 Paleocene, the majority are evidently recent immigrants from some 

 region not yet identified. From the Wasatch onward, the develop- 

 ment of many phyla may be traced in almost unbroken continuity, 

 though from time to time the record is somewhat obscured by 

 migrations from the Old World and South America. As a rule, 

 however, it is easy to distinguish between the immigrant and the 

 indigenous elements of the fauna. 



1 In the Miocene beds of Steinheim, Wiirtemberg, occur countless fresh-water shells, 

 which show numerous lines of modification, but these have been very differently inter- 

 preted by different writers. 



2 D. H. Scott, Studies in Fossil Botany, p. 524. London, 1900. 



