the carboniferous period. Other
groups which once played a great _role_, are now wholly extinct; for
instance, the trilobites of the primary, the sauria of the secondary, the
nummulites of the tertiary periods. Now, all these modifications of
geological progress would entirely correspond to the idea of a pedigree to
which the descent theory traces back the whole abundance of forms of
organisms. As soon as we seriously accept the idea of a pedigree, each of
the two organic kingdoms would throughout form for its classes and species
not only one single straight line of descent, but a tree, the branches of
which are again ramified in a manifold way; a tree on which single
branches--as perhaps that of the class of birds--may leave the main-stem or
a main-branch, possibly being a branch destined to a higher development,
and on that account held back in the process of development; a tree,
finally, on which also branches and twigs can wholly or partly die off, as
those of the extinct or reduced groups of organisms. {69}

From the point where the geological formations approach the present time,
_plant_ and _animal geography_ also assists _geology_ in increasing the
weight of the reasons for an origin of organisms through descent. With the
tertiary period, the fauna and flora of the globe, which in former periods
had a nearly uniform character all over the earth and showed no climatic
differences, begin to separate according to climate, zones, and greater
continents. This separation becomes distinctly evident in the middle
tertiary formations, the Miocene, and much more distinctly in the higher
tertiary formations, the Pliocene. The animals, especially the higher
vertebrates, of the Pliocene formation on each continent or each larger
group of islands, correspond very closely to the now living animals of the
same geographical limit, with the exception of being generally of a much
larger size. The Pliocene animal world of mammalia of the three old
continents, for instance, corresponds exactly, through all its orders, to
the present fauna of Europe, Asia and Africa; and that on an average it was
built up more stupendously than that of to-day, we can see from the
cave-bear and the mammoth. South America is the home of a peculiar order of
mammalia--of the edentata, to which belong the sloth, the armadillo, and
the like. All its predecessors are to be found also in the Pliocene strata
of South America, and only there; and mostly in giga