Fossil Mammals 189 
wished, different observers will put different interpretations upon 
it, as in the notorious case of the Steinheim shells’. The ludicrous 
discrepancies which often appear between the phylogenetic “ trees” 
of various writers have cast an undue discredit upon the science and 
have Jed 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”.” 
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. 
2D. H, Scott, Studies in Fossil Botany, p. 524. London, 1900. 
