s 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." (D.H. Scott, "Studies in
Fossil Botany", page 524. London, 1900.)

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 continental
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 preservation. 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 development 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.

From their gregarious habits and individual abundance, the history of
many hoofed animals is preserved with especial clearness. So well known
as to have become a commonplace, is the phylogeny of the horses, which,
contrary to all that would have been expected, ran the greater part of
its course in North America. So far as it has yet been traced, the line
begins in the lower Eocene with the