A A. A primitive Equid, Orohipnus sp. B. A primitive Titanothere, Palæosyops paludosus. From specimens in the American Museum of Natural History. Internal view of lower teeth. The circles mark the cusps which appear independently in the different phyla ; they are at first barely visible but increase in size in successive geological levels. TOP VIEW OF THREE SKULLS OF EOCENE TITANOTHERES ILLUSTRATING IVELY. (From specimens in the American Museum of Natural History.) A. Palæosyops major, brachycephalic. B. Manteocer as manteocer as, mesaticephalic. (different from that of the tapirs, rhinoceroses, or horses) apparently tincture and condition many things which happen in the evolution of this group. There are forty-four grinding teeth altogether; twelve of these teeth (the molars) early attain their final form, but are destined through family kinship to lose certain characters and to change their proportions through generic kinship; twelve others (the premolars) have not attained their final form, but gradually do so through the origin of from forty to forty-eight new characters, each of which appears to arise through an unknown law of hereditary predisposition, which operates alike, through ordinal kinship, not only in the titanotheres but in all other odd-toed or perissodactyl mammals to which the titanotheres are related. Changes of proportion in the skull, whether toward breadth (brachycephaly), or toward length (dolichocephaly), affect the form of the grinders as a whole and thus the birth-form of each of these new cusps. The immediate cause of changes of proportion is not interpreted as due to hereditary predisposition, because in teeth, in skull, in foot and limb, and even in horns each generic branch or phylum from the original stem forms of titanotheres acquires its own proportions. Thus changes of proportion are interpreted rather as immediately affected by ontogeny, by the mechanics of use and disuse, by an environment which favors some rather than other proportions, but especially by the selection of variations in proportion which coincide with the needs of the phylum.' No abrupt variations (mutations) have been observed in the evolution of the titanotheres, but this in no way renders it inconceivable that skeletal mutations in the De Vries sense have produced new races in certain phyla. The addition or loss of a vertebra in the sacral region, which appears to distinguish certain titanothere phyla, may be a case of such sudden inheritable mutations. Independently in four or five Eocene branches of the titanothere stock the horn rudiments very gradually arise, apparently through hereditary predisposition or family kinship, as rectigradations, at the junction of the nasal and frontal bones. As in the case of the cusps, the shape of these horn rudiments is from the first conditioned by the respective breadth (brachycephaly) or length (dolichocephaly) of the skull. The branches or sub-phyla become more and more sharply distinguished from each other by increasing brachycephaly or dolichocephaly, brachypody or dolichopody, apparently through congenital variations of proportion accumulated by selection and guided by ontogeny through organic selection." The animals belonging to It is important to note, on the authority of Professor Castle, that proportions of the skeleton and probably of the teeth are not inherited as distinct "unit characters." Inheritance of bone size and shape seems to be as a rule regularly blended by interbreeding and without subsequent Mendelian splitting. or these branches appear to have chosen their own local environments, whether in localities favorable to grazing or to browsing, and in turn congenital changes of proportion would be favored by selection if in the right direction. The transformation into brachycephaly and dolichocephaly is brought about through independent changes of proportion in every bone of the skull, as ascertained by exact comparative measurements. A trend once established in either direction seems to constitute a sort of "hereditary momentum " predisposition, which leads to great extremes of brachycephaly, on the one hand, or dolichocephaly on the other, as shown in the accompanying cut. The rudimentary horns, at first barely noticeable as the faintest convexities of the skull invariably appearing at the junction of the frontals and nasals, and produced by a thickening of the cellular spaces, are first observed of equal size in the males and females; later they become more prominent in the males than in the females; finally they assume vast proportions in the males and present an arrested development in the females. At the summit of the Eocene the extreme dolichocephalic and brachycephalic phyla die out, and in the Oligocene a new series of phyla arise. Among these the long-horned forms appear through selection to develop the horns at the expense of other characters, the males with the longest horns probably securing the most females and becoming the chief breed |