Page:Popular Science Monthly Volume 27.djvu/218

204 the hind-limbs with the rest of the skeleton. The pelvic arch is thus almost universally present, but of the limb proper there is, as far as is yet known, not a vestige in any of the large group of toothed whales, not even in the great Cachalot or sperm whale, although it should be mentioned that it has never been looked for in that animal with any sort of care. With regard to the whalebone whales, at least to some of the species, the case is different. In these animals there are found, attached to the outer and lower side of the pelvic bone, other elements, bony or only cartilaginous as the case may be, clearly representing rudiments of the first and in some cases the second segment of the limb, the thigh or femur, and the leg or tibia.

We have here a case in which it is not difficult to answer the question before alluded to, often asked with regard to rudimentary parts: Are they disappearing, or are they incipient organs? We can have no hesitation in saying that they are the former. All we know of the origin of limbs shows that they commence as outgrowths upon the surface of the body, and that the first-formed portions are the most distal segments. The limb, as proved by its permanent state in the lowest vertebrates, and by its embryological condition in higher forms, is at first a mere projection or outward fold of the skin, which, in the course of development, as it becomes of use in moving or supporting the animal, acquires the internal framework which strengthens it and perfects its functions. It would be impossible, on any theory of causation yet known, to conceive of a limb gradually developed from within outward. On the other hand, its disappearance would naturally take place in the opposite direction.

We turn next to what the researches of paleontology teach of the past history of the order. Unfortunately, this does not at present amount to very much. We know nothing of their condition, if they existed, in the Mesozoic age. Even in the cretaceous seas not a fragment of any whale or whale-like animal has been found. The earliest Cetaceans, of whose organization we have any good evidence, are the Zeuglodons of the Eocene formations of North America. These were creatures whose structure, as far as we know it, was intermediate between that of the existing sub-orders of whales. In fact, Zeuglodon is precisely what we might have expected a priori an ancestral form of whales to have been. From the middle Miocene period fossil Cetacea are abundant, and distinctly divided into the two groups now existing. The Mystacocetes, or whalebone whales, of the Miocene seas, were, as far as we know now, only Balænoptera, some of which were more generalized than any now existing. In the shape of the mandible also, Van Beneden discerns some approximation to the Odontocetes. Right whales (Balæna) have not been found earlier than the Pliocene period, and it is interesting to note that, instead of the individuals diminishing in balk as we approach the times we live in, as with many other groups of animals, the contrary has been the case, no known extinct species