Fig. 39.—Epitome of the evolution of a cusped tooth. 1, Reptile; 2, Dromatherium; 3, Microconodon; 4, Spalacotherium; me, metaconid; pa, paraconid; pr, protoconid; 5, Amphitherium. (After Osborn.)
That such a series can be traced is an undoubted fact. Every stage exists, or has existed. But whether the stages can be connected or not is quite another question. It is by three main lines of argument that the view here sketched out in brief is supported. In the first place, the tracing of the pedigrees of many groups of mammals has met with very considerable success; and it is clear that as we pass from the living Horse and Rhinoceros, with their complicated molars, to their forerunners, we find that both can be referred to a primitive Ungulate molar with but six cusps. Going still further back to the lowest Eocene and ancestral type as it appears, Euprotogonia, we still find in the molar tooth the sextubercular plan of structure. We can hardly get further back in the evolution of the Perissodactyles with any probability of security. On the other hand, many facts point to a fundamental relationship between the primitive Ungulates and the early Creodonts. The latter frequently show plainly tritubercular molars. Such Ungulates as Euprotogonia and Protogonodon, though sex- or quinque-tubercular as to their molars, have a distinctly prevailing trituberculism, when the size and importance of three of the cusps is taken into account. But this
lacks finality as a convincing proof of the tritubercular tooth as a primitive Ungulate tooth.
Professor Osborn has ingeniously utilised certain deviations from the normal type of tooth structure (for the group) in favour of his strongly-urged opinions. If the stages of development have been as he suggests, a retrogression would naturally be in the inverse order; thus the "apparently 'triconodont' lower molar of Thylacinus" may be interpreted as a retrogression from a tritubercular tooth. In the same way may be explained the triconodont teeth of Seals and of the Cetacean Zeuglodon. Finally, the modern Toothed Whales have retrograded into "haplodonty."
Embryological evidence has also been called in, and with some success, to contribute towards the proof of the tritubercular theory of teeth. Taeker has shown that in the Horse and the Pig, and some other Ungulates, there is first of all a single hillock or cusp, and that later the additional cones arise separately. An apparent stumbling-block raised by these investigations is that it is not always the protocone or its equivalent in the upper jaw which arises first, as it obviously ought to do phylogenetically. This, however, is not a final argument in either direction. We know from plenty of examples that ontogenetic processes sometimes do not correspond in their order with phylogenetic changes. Thus in the mammalian heart the ventricle divides before the auricle; and of coarse, phylogenetically, the reverse ought to occur, since a divided auricle precedes a divided ventricle. This method of development has, moreover, been interpreted otherwise. It has been held to signify that the complex teeth of mammals are indeed derived from simple cones but by the fusion of a number of those cones.
On the other hand there are the claims of the multitubercular theory of the origin of mammalian teeth to be considered. The palaeontological evidence has been already, to some extent, utilised. The occurrence of such teeth among the possible forerunners of mammals, and in some of the most primitive types of Mammalia, has been referred to. Señor Ameghino dwells upon the sextubercular condition of many primitive mammals even belonging to the Eutheria. In a recent communication[[30]] he attempts to identify six tubercles in the molars of types belonging to a
variety of Orders. The same condition, as has been noted, characterises that ancient Ungulate form Euprotogonia. Even where the teeth seem at first sight to be tritubercular a detailed study shows traces of otherwise vanished cusps.
It must be remembered in basing arguments upon the early Jurassic and Cretaceous mammals, that our knowledge of them mainly depends upon lower jaws, the teeth of which are usually simpler in pattern than those of the upper jaws. Moreover, another fact, not always insisted upon, must not be lost sight of. In many of those creatures the jaws were of small size, and yet accommodated a large series of molar teeth. Amphitherium, for example, had six molar teeth, and five is a number frequently met with. As the teeth are so numerous and the jaws so small it seems reasonable to connect the simplicity of the structure of the teeth with the need for crowding a number together. The same argument may partly account for the superabundant teeth of many Toothed Whales. It is true that the Manatee has very numerous grinders which are yet complex; but then in this animal there is a succession, and the jaw does not hold at a given time the entire series, with which it is provided in relays. On the other hand, where there are few molars they are often of the multitubercular type, or at least approach it; of this the Multituberculate Polymastodon is a good example; so, too, the molars of Hydrochoerus, and of many other Rodents.
It is well known that the fourth deciduous molar of the upper jaw, which is replaced by a permanent premolar in the fully adult animal, is of a more complex structure than its successor. This may indeed be extended to premolars earlier in the series. In the Dog "the second and first milk molars closely resemble the third and second premolars"; now the milk premolars belong evidently to the same dentition as the permanent molars, and they are earlier teeth than the later-developed replacing teeth. It is therefore significant that these earlier teeth should be more cuspidate than the later teeth. It tells distinctly in favour of the simplification as opposed to the complication of teeth in time, in the groups concerned.
These facts may possibly be applied in explanation of the simple teeth of some of the Jurassic and Cretaceous mammals. It has been mentioned that absolute trituberculy is exceedingly rare among those ancient creatures; more generally there are to