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 gigantic, but otherwise completely related, forms. New Zealand has no indigenous mammalia, but in their place great cursorial birds with but rudimentary wings. Exactly the same thing is found by geology in its tertiary and post-tertiary strata: nowhere a mammal, but gigantic birds with rudimentary {70} wings, down to the dinornis, which probably died out in man's time. New Holland has merely marsupial and some monotrematous, but no placental, mammalia; even its tertiary strata give no placental mammalia, but marsupialia, in analogy with all living genera, herbivorous, and carnivorous. Indeed, the analogy goes so far that the same line which through the Indian Archipelago separates the present Australian animal and plant world from the Asiatic,