There are no better examples of such success today, nor has there been in all the geological ages, so far as we know, more perfect examples of the adaptation of air-breathing animals to an aquatic life than the great whalebone whales. In Eocene times their ancestors were walking and running land animals; of that there can be not the slightest doubt, since we cannot conceive, as did the older naturalists, of their direct descent from the fishes while having all the essential structure of mammals, i.e., lungs, circulatory system, manner of breeding and rearing the young, etc. Of the living whales, or Cetacea, there are now in existence two very distinct types, so different from each other that some have supposed them to have been evolved from different types of land mammals. One of these is best exemplified by the great baleen whale, having a broad, short head and no teeth. It feeds upon crustaceans chiefly, which are strained from the water by the great fringe or net of “whalebone.” The other type is seen in the porpoise or dolphin. These cetaceans have numerous, pointed and recurved teeth, which they use as did many of the reptiles, hereinafter described, for the seizure and retention of fishes and other swimming animals. So great have been the changes in all these cetaceans, in the adaptation to an aquatic life, that we are almost at a loss to conjecture from what kinds of land animals they have descended. The great zeuglodont whales of early Tertiary times have long been thought to be a sort of connecting link between them and their land ancestors, and it is still probable that they were. The forms of zeuglodont whales that have been discovered in Africa within recent years bear so much resemblance in their skull and teeth to the contemporary carnivores, that many paleontologists think, with good reason, that they were descended from them, that is, from the ancestors of all our dogs, cats, weasels, bears, etc., of modern times. And we have much reason to believe that future discoveries will bring further and more decisive proof of their origin before many years have elapsed. The modern Sirenia, the dugongs and manatees, exclusively aquatic mammals, which feed upon seaweeds at the bottoms of shallow bays and harbors, or in the mouths of rivers, are now known, practically with certainty, to be the descendants in these same African regions of the earliest ancestors of our sheep, oxen, and horses, known so certainly that they are often classed with them, or at least with the elephants, which approach them in their ancestral line even more closely.