“The origin of the Cetacea,” he wrote, “is at present involved in much obscurity. They present no signs of closer affinity to any of the lower classes of vertebrates than do many other members of their own class. Indeed in all that essentially distinguishes a mammal from the oviparous vertebrates, whether in the osseous, nervous, reproductive, or any other system, they are as truly mammalian as any other group. Any supposed marks of inferiority, as absence of limb-structure, of hairy covering, of lachrymal apparatus, etc., are obviously modifications (or degradations, as they may be termed) in adaptation to their special mode of life. The characters of the teeth of _Zeuglodon_ and other extinct forms, and also of the fœtal Mystacocetes, clearly indicate that they have been derived from mammals in which the heterodont type of dentition was fully established. The steps by which a land mammal may have been modified into a purely aquatic one are indicated by the stages which still survive among the Carnivora in the Otariidæ and in the true seals. A further change in the same direction would produce an animal somewhat resembling a dolphin; and it has been thought that this may have been the route by which the Cetacean form has been developed. There are, however, great difficulties in the way of this view. Thus if the hind-limbs had ever been developed into the very efficient aquatic propelling organs they present in the seals, it is not easy to imagine how they could have become completely atrophied and their function transferred to the tail. So that, from this point of view, it is more likely that whales were derived from animals with long tails, which were used in swimming, eventually with such effect that the hind-limbs became no longer necessary. The powerful tail, with its lateral cutaneous flanges, of an American species of otter (_Lutra brasiliensis_) may give an idea of this member in the primitive Cetaceans. But the structure of the Cetacea is, in so many essential characters, so unlike that of the Carnivora, that the probabilities are against these orders being nearly related. Even in the skull of the _Zeuglodon_, which has been cited as presenting a great resemblance to that of a seal, quite as many likenesses may be traced to one of the primitive Pig-like Ungulates (except in the purely adaptive character of the form of the teeth) while the elongated larynx, complex stomach, simple liver, reproductive organs, both male and female, and fœtal membranes of the existing Cetacea, are far more like those of that group than of the Carnivora. Indeed, it appears probable that the old popular idea which affixed the name of ‘Sea-Hog’ to the porpoise, contains a larger element of truth than the speculations of many accomplished zoologists of modern times. The fact that _Platanista_, which, as mentioned above, appears to retain more of the primitive characteristics of the group than any other existing form, and also the