The second family of the order, the †Trigonostylopidæ, did not survive beyond the Eocene and is so imperfectly known that any account of it would be to small profit.
As stated above, the †Astrapotheria were an isolated group and their relationships are problematical and are likely to remain so pending the discovery of much more complete specimens of the various genera which made up the series. I am inclined to the opinion, however, that all of the indigenous groups of South American ungulates, which inhabited that continent before the great immigration from the north, were derivatives of the same stock and more nearly related to one another than to any of the orders which lived in other regions.
In looking over the labyrinth of ungulate history, as recorded by the fossils, certain facts stand out clearly, while others are still very obscure. It is like trying to trace the plan of vast and complicated ruins, which here are deeply buried in their own débris, there are fully exposed and in another place are swept away so completely that hardly a trace remains. But the problem is far more complex than any which can be presented by buildings, for the factor of repeated migrations from continent to continent comes in to obscure the evidence. Had each of the great land areas received its original stock of early mammals and then been shut off from communication with any other, many of the difficulties would be removed, but the story would lose half its interest.
Within the limits of the family, giving to that group the broad and elastic definition which has hitherto been employed, we have repeatedly found it feasible to construct a phylogenetic series which very nearly represents the steps of structural modification as they occurred in time. Much less frequently is it possible to trace allied families to their common starting point, and, so far as the hoofed animals are concerned, in no case have we yet succeeded in doing this for the separate orders. The obstacle lies in the fact that the ordinal groups were already distinct, when they made their first appearance in the known and accessible records, and the hypothetical ancestors common to them all, or to any two of them, are to be sought in regions of which we know little or nothing. Nevertheless, certain legitimate inferences may be drawn from the available evidence. It remains to be proved whether the assemblage of hoofed mammals, as a whole, was of single or multiple origin. Have all ungulates been derived from a common stock, or did they arise independently from several groups of clawed mammals? While the records cannot be followed back to the point, or points, of origin of the various orders, yet it is a noteworthy fact that, between several of them, the differences grow less marked as the more ancient members are reached, as though they were converging to a common term; others again show little such approximation, and the most probable conclusion from the evidence now at hand is that the ungulate assemblage is composed of several independent series.
One such series is that of the Hyracoidea and Proboscidea, to which Dr. Schlosser has given the name “Subungulata,” and has pointed out its relationship to the †Condylarthra, which, however, is not a close one and may be illusory. Another apparently natural group is that of the peculiarly South American forms, the †Toxodontia, with its four suborders, the †Litopterna and the †Astrapotheria, which all appear to be traceable to closely allied families in the Eocene, whose teeth strongly suggest derivation from the †Condylarthra; but the material does not permit any positive statements. The Artiodactyla and Perissodactyla have so many similarities that they have always been regarded as closely related groups, but the distinction between them was almost as sharply drawn in their most ancient known members as it is to-day, and there was no distinct tendency to converge back into a common stem. Their mutual relationships are thus obscure, but the Perissodactyla, at least, seem to be derivable from a †condylarthrous ancestry.
The †Condylarthra, as a whole, were by far the most primitive of the ungulates, which they connected with the clawed mammals. None of the genera yet discovered can be regarded as ancestral to any of the higher orders, but it is entirely possible that in the upper Cretaceous period the †Condylarthra were spread over all the continents, except Australia, and that from them the other ungulate orders arose in different regions. At all events, the †Condylarthra show how the transition from clawed to hoofed types may have occurred and perhaps actually did so, but it would be premature to affirm this.
CHAPTER XIV
HISTORY OF THE CARNIVORA
The story of the hoofed mammals, as sketched in brief outline in the preceding chapters (VIII-XIII), is a curious mixture of relatively full and satisfactory paragraphs, with scanty, broken and unintelligible ones, not to mention those which have not yet been brought to light at all. With all its gaps and defects, which inhere in the nature of things, the history of the various ungulate series is the best that the palæontology of mammals has to offer and constitutes a very strong and solid argument for the theory of evolution. For the Carnivora the story is less complete and for obvious reasons. Individual abundance was a very large factor in determining the chances of preservation in the fossil state for any given species, and, as a rule, whole skeletons are found only when the species was fossilized in large numbers. In any region the Carnivora are less numerous than the herbivora upon which they prey, and while most ungulates live in larger or smaller herds, the carnivores are mostly solitary.