Page:Popular Science Monthly Volume 15.djvu/261

Rh common with the giraffe, the elongation of whose neck is produced not by introduction of new vertebræ, but by the great development of the normal number, seven. The manatees, however, present a very remarkable exception to this most general of rules, in that they possess only six vertebræ in their necks. The only other exceptions to the rule of seven, as the normal number of neck-vertebræ in quadrupeds, are found in one species of sloth which has six vertebræ like the manatee, and in another kind of sloth which possesses nine. Then, also, the manatees possess a heart of very curious conformation, its apex or tip being widely cleft or divided—a feature much more plainly marked in these animals than in the elephants and seals, whose hearts, anatomically speaking, are also divided. The manatees possess well-developed molars or grinding teeth, but have no front teeth in the adult state. Like the whalebone whale, however, the young manatee has front teeth, these again disappearing before birth, and presenting us once more with examples of rudimentary organs which possess a reference "to a former state of things."

What evidence is at hand respecting the remote ancestors of the whales and their neighbors? is a question which may form a fitting conclusion to these brief details of the family history of the group. The geological evidence shows us that the whales are comparatively "recent" forms, speaking geologically, and dealing—notwithstanding the word "recent"—with very remote and immense periods of time. Among the oldest fossil whales we find one form in particular (Zeuglodon) which had teeth of larger kind than are possessed by any living whale, this creature being by some authorities regarded as linking the whales with the seals. The fossil remains of Zeuglodon and its neighbors first occur in Eocene rocks—that is, in the oldest formations of the Tertiary series, and in rocks of relatively "recent" nature. These remarkable creatures were as gigantic as their living representatives. One species is known to have attained a length of seventy feet. Their remains are of such frequent occurrence in the "Jackson Beds" of the United States, that Professor Dana remarks: "The large vertebræ, some of them a foot and a half long and a foot in diameter, were formerly so abundant over the country in Alabama, that they were used for making walls, or were burned to rid the fields of them." The teeth of this curious monster of the vasty Eocene deep were of two kinds, and included front teeth of conical shape, and grinders or molars; the latter exhibiting a striking peculiarity in that they were formed each of two halves, or teeth united by their crowns, but separated at their roots. Zeuglodon appears to connect the whales and their neighbors with the seals and walruses, and thus in one sense may be said to constitute, if not a "missing link," at least an intermediate form of anomalous kind, when viewed relatively to the existing cetaceans. According to the geological evidence at hand, we may assume that the modifications which have produced the existing whales and