Popular Science Monthly/Volume 27/June 1885/Whales, Past and Present
|WHALES, PAST AND PRESENT.|
By Professor W. H. FLOWER, F. R. S.
FEW natural groups present so many remarkable illustrations of several of the most important general laws which appear to have determined the structure of animal bodies as that of the whales. We find the effects of the two opposing forces—that of heredity or conformation to ancestral characters, and that of adaptation to changed environment, whether brought about by the method of natural selection or otherwise—distinctly written in almost every part of their structure. Scarcely anywhere in the animal kingdom do we see so many cases of the persistence of rudimentary and apparently useless organs, those marvelous and suggestive phenomena which at one time seemed hopeless enigmas, causing despair to those who tried to unravel their meaning, but now eagerly welcomed as beacons of true light, casting illuminating beams upon the dark paths through which the organism has traveled on its way to reach the goal of its present condition of existence.
It is chiefly to these rudimentary organs of the Cetacea and to what we may learn from them that I propose to call your attention. In each case the question may well be asked, Are they survivals, remnants of a past condition, become useless owing to change of circumstances and environment; or are they incipient structures, beginnings of what may in future become functional and important parts of the economy?
The term "whale" is commonly but vaguely applied to all the larger and middle-sized Cetacea, and, though such smaller species as the dolphins and porpoises are not usually spoken of as whales, they may to all intents and purposes of zoölogical science be included in the term. Taken all together the Cetacea constitute a distinct and natural order of mammals, characterized by their aquatic mode of life and external fish-like form. The body is fusiform, passing anteriorly into the head without any distinct constriction or neck, and posteriorly tapering off gradually toward the extremity of the tail, which is provided with a pair of lateral pointed expansions of skin supported by dense fibrous tissue, called "flukes," forming together a horizontally placed, triangular propelling organ. The fore-limbs are reduced to the condition of flattened ovoid paddles, incased in a continuous integument, showing no external sign of division into arm, fore-arm, and hand, or of separate digits, and without any trace of nails. There are no vestiges of hind-limbs visible externally. The general surface of the body is smooth and glistening, and devoid of hair. In nearly all species a compressed median dorsal fin is present. The nostrils open separately or by a single crescentic valvular aperture, not at the extremity of the snout, but near the vertex.
Animals of the order Cetacea abound in all known seas, and some species are inhabitants of the larger rivers of South America and Asia. Their organization necessitates their life being passed entirely in the water, as on g the land they are absolutely helpless; but they have to rise very frequently to the surface for the purpose of respiration. They are all predaceous, subsisting on living animal food of some kind. One genus alone (Orca) eats other warm-blooded animals, as seals and even members of its own order, both large and small. Some feed on fish, others on small floating Crustacea, pteropods, and medusa?, while the staple food of many is constituted of the various species of Cephalopods. With some exceptions the Cetacea generally are timid, inoffensive animals, active in their movements, sociable and gregarious in their habits.
Among the existing members of the order there are two very distinct types the toothed whales, or Odontoceti, and the baleen (whale-bone) whales, or Mystacoceti, which present throughout their organization most markedly distinct structural characters, and have in the existing state of nature no transitional forms.
The problem of the origin of the Cetacea and their relations to other forms of life is at present involved in the greatest obscurity. They present no more signs of affinity with any of the lower classes of vertebrated animals than do many of the members of their own class. Indeed, in all that essentially distinguishes a mammal from one of the oviparous vertebrates, they are as truly mammalian as any, even the highest, members of the class. Any supposed signs of inferiority are simply modifications in adaptation to their peculiar mode of life. In the present state of our knowledge, the Cetacea are absolutely isolated, and little satisfactory reason has ever been given for deriving them from any one of the existing divisions of the class more than from any other. The question has indeed often been mooted whether they have been derived from land mammals at all, or whether they may not be the survivors of a primitive aquatic form which was the ancestor not only of the whales, but of all the other members of the class. The materials for—I will not say solving—but for throwing some light upon this problem, must be sought for in two regions—in the structure of the existing members of the order, and in its past history, as revealed by the discovery of fossil remains. In the present state of science it is chiefly on the former that we have to rely. One of the most obvious external characteristics by which the mammalia are distinguished from other classes of vertebrates is the more or less complete clothing of the surface by hair. The Cetacea alone appear to be exceptions to this generalization. Their smooth, glistening exterior is, in the greater number of species, at all events in adult life, absolutely bare, though the want of a hairy covering is compensated for functionally by peculiar modifications of the structure of the skin itself, the epidermis being greatly thickened, and a remarkable layer of dense fat closely incorporated with the tissue of the derm or true skin; modifications admirably adapted for retaining the warmth of the body, without any roughness of surface which might occasion friction and so interfere with perfect facility of gliding through the water. Close examination, however, shows that the mammalian character of hairiness is not entirely wanting in the Cetacea, although it is reduced to a most rudimentary and apparently functionless condition.
In the organs of the senses the Cetacea exhibit some remarkable adaptive modifications of structures essentially formed on the mammalian type, and not on that characteristic of the truly aquatic vertebrates, the fishes.
The modifications of the organs of sight do not so much affect the eyeball as the accessory apparatus. To an animal whose surface is always bathed with fluid, the complex arrangement which mammals generally possess for keeping the surface of the transparent cornea moist and protected, the movable lids, the nictitating membrane, the lachrymal gland, and the arrangements for collecting and removing the superfluous tears when they have served their function can not be needed, and hence we find these parts in a most rudimentary condition or altogether absent. In the same way the organ of hearing in its essential structure is entirely mammalian, having not only the sacculi and semicircular canals common to all but the lowest vertebrates, but the cochlea, and tympanic cavity with its ossicles and membrane, all, however, buried deep in the solid substance of the head; while the parts specially belonging to terrestrial mammals, those which collect the vibrations of the sound traveling through air, the pinna and the tube which conveys it to the sentient structures within, are entirely or practically wanting. Of the pinna or external ear there is no trace.
The organ of smell, when it exists, offers still more remarkable evidence of the origin of the Cetacea. In fishes this organ is specially adapted for the perception of odorous substances permeating the water; the terminations of the olfactory nerves are spread over a cavity near the front part of the nose, to which the fluid in which the animals swim has free access although it is quite unconnected with the respiratory passages. Mammals, on the other hand, smell substances with which the atmosphere they breathe is impregnated; their olfactory nerve is distributed over the more or less complex foldings of the lining of a cavity placed in the head, in immediate relation to the passages through which air is continually driven to and fro on its way to the lungs in respiration, and therefore in a most favorable position for receiving impressions from substances floating in that air. The whalebone whales have an organ of smell exactly on the mammalian type, but in a rudimentary condition. In the more completely modified Odontocetes the olfactory apparatus, as well as that part of the brain specially related to the function of smell, is entirely wanting, but in both groups there is not the slightest trace of the specially aquatic olfactory organ of fishes. Its complete absence and the vestiges of the aerial organ of land mammals found in the Mystacocetes are the clearest possible indications of the origin of the Cetaceæ from air breathing and air-smelling terrestrial mammalia. With their adaptation to an aquatic mode of existence, organs fitted only for smelling in air became useless, and so have dwindled or completely disappeared. Time and circumstances have not permitted the acquisition of anything analogous to the special aquatic smelling apparatus of fishes, the result being that whales are practically deprived of whatever advantage this sense may be to other animals.
All the Cetacea present some traces of teeth, which in structure and mode of development resemble those of mammals, and not those of the lower vertebrated classes, but they are always found in a more or less imperfect state.
The meaning and utility of some of the strange modifications in the dentition of whales it is impossible, in the imperfect state of our knowledge of the habits of the Cetacea, to explain, but the fact that in almost every case a more full number of rudimentary teeth is present in early stages of existence, which either disappear, or remain as concealed and functionless organs, points to the present condition in the aberrant and specialized forms as being one derived from the more generalized type, in which the teeth were numerous and equal.
Fig. 3.—Toothed Whale, or Spermaceti Whale.
The Mystacocetes, or whalebone whales, are distinguished by entire absence of teeth, at all events after birth. But it is a remarkable fact, first demonstrated by Geoffroy Saint-Hilaire, and since amply confirmed by Cuvier, Eschricht, Julin, and others, that in the fetal state they have numerous minute calcified teeth lying in the dental groove of both upper and lower jaws. These attain their fullest development about the middle of fetal life, after which period they are absorbed, no trace of them remaining at the time of birth. Their structure and mode of development have been shown to be exactly those characteristic of ordinary mammalian teeth. It is not until after the disappearance of these teeth that the baleen, or whalebone, makes its appearance. This remarkable structure, though only a modification of a part existing in all mammals, is, in its specially developed condition as baleen, peculiar to one group of whales.
Baleen consists of a series of flattened, horny plates, several hundred in number, on each side of the palate, separated by a bare interval along the middle line. They are placed transversely to the long axis of the palate, with very short spaces between them. Each plate or blade is somewhat triangular in form, with the base attached to the palate, and the apex hanging downward. The outer edge of the blade is hard and smooth, but the inner edge and apex fray out into long, bristly fibers, so that the roof of the whale's mouth looks as if covered with hair, as described by Aristotle. The blades are longer near the middle of the series, and gradually diminish near the front and back of the mouth. The horny plates grow from a dense, fibrous, and highly vascular matrix, which covers the palatal surface of the maxillae, and which sends out lamellar processes, one of which penetrates the base of each blade. Moreover, the free edge of these processes is covered with very long, vascular, thread-like papillæ, one of which forms the central axis of each of the hair-like epidermic fibers of which the blade is mainly composed. The blades are supported and bound together, for a certain distance from their base, by a mass of less hardened epithelium, secreted by the surface of the palatal membrane or matrix of the whale-bone in the intervals of the lamellar processes. This is the "intermediate substance" of Hunter, the "gum" of the whalers.
The function of the whalebone is to strain the water from the small marine mollusks, crustaceans, or fish upon which the whales subsist. In feeding they fill the immense mouth with water containing shoals of these small creatures, and then, on their closing the jaws and raising
Fig. 4.—Whalebone Whale, or Greenland Whale.
the tongue, so as to diminish the cavity of the mouth, the water streams out through the narrow intervals between the hairy fringe of the whale-bone blades, and escapes through the lips, leaving the living prey to be swallowed. Almost all the other structures to which I am specially directing your attention are in a more or less rudimentary state in the Cetacea; the baleen, on the other hand, is an example of an exactly contrary condition, but an equally instructive one, as illustrating the mode in which Nature works in producing the infinite variety we see in animal structures. Although appearing at first sight an entirely distinct and special formation, it evidently consists of nothing more than the highly modified papillæ of the lining membrane of the mouth, with an excessive and cornified epithelial development.
The bony palate of all mammals is covered with a closely-adhering layer of fibro-vascular tissue, the surface of which is protected by a coating of non-vascular epithelium, the former exactly corresponding to the derm or true skin, and the latter to the epiderm of the external surface of the body. Sometimes this membrane is perfectly smooth, but it is more often raised into ridges, which run in a direction transverse to the axis of the head, and are curved with the concavity backward; the-ridges, moreover, do not extend across the middle line, being interrupted by a median depression or raphé. Indications of these ridges are clearly seen in the human palate, but they attain their greatest development in the Ungulata.
Though the early stages by which whalebone has been modified from more simple palate structures are lost to our sight, the conditions in which it now exists in different species of whales show very marked varieties of progress, from a simple, comparatively rudimental and imperfect condition, to what is perhaps the most wonderful example of mechanical adaptation to purpose known in any organic structure.
In the rorquals or fin-whales (genus Balænoptera'), found in almost ail seas, the largest blades in an animal of seventy feet in length do not exceed two feet in length, including their hairy terminations; they are in most species of a pale horn color, and their structure is coarse and inelastic, separating into thick, stiff fibers, so that they are of no value for the ordinary purposes to which whalebone is applied in the arts. These animals feed on fish of considerable size, from herrings up to cod, and for foraging among shoals of these creatures the construction of their mouth and the structure of their baleen are evidently sufficient. This is the type of the earliest known extinct forms of whales, and it has continued to exist, with several slight modifications, to this day, because it has fulfilled one purpose in the economy of Nature. Other purposes for which it was not sufficient have been supplied by gradual changes taking place, some of the stages of which are seen in the intermediate conditions still exhibited in the Megaptera and the Atlantic and southern right whales.
In the Greenland right whale of the circumpolar seas, the Bowhead of the American whalers (Balæna mysticetus), all the peculiarities which distinguish the head and mouth of the whales from other mammals have attained their greatest development. The head is of enormous size, exceeding one third of the whole length of the creature. The cavity of the mouth is actually larger than that of the body, thorax, and abdomen together. The upper jaw is very narrow, but greatly arched from before backward, to increase the height of the cavity and allow for the great length of the baleen; the enormous rami of the mandibles are widely separated posteriorly, and have a still further outward sweep before they meet at the symphysis in front, giving the floor of the mouth the shape of an immense spoon. The baleen-blades attain the number of three hundred and fifty or more on each side, and those in the middle of the series have a length of ten or even twelve feet. They are black in color, fine and highly elastic in texture, and fray out at the inner edge and ends into long, delicate, soft, almost silky, but very tough hairs.
Fig. 5.—Skull of Greenland Whale, showing Whalebone.
How these immensely long blades, depending vertically from the palate, were packed into a mouth the height of which was scarcely more than half their length, was a mystery not solved until a few years ago. Captain David Gray, of Peterhead, at my request, first gave us a clear idea of the arrangement of the baleen in the Greenland whale, and showed that the purpose of its wonderful elasticity was not primarily at least the benefit of the corset and umbrella makers, but that it was essential for the correct performance of its functions. It may here be mentioned that the modification of the mouth-structure of the right whale is entirely in relation to its food. It is by this apparatus that it is enabled to avail itself of the minute but highly nutritious crustaceans and pteropods which swarm in immense shoals in the seas it frequents. The large mouth enables it to take in at one time a sufficient quantity of water filled with these small organisms, and the length and delicate structure of the baleen make it an efficient strainer or hair sieve by which the water can be drained off. If the baleen were, as in the rorquals, short and rigid, and only of the length of the aperture between the upper and lower jaws when the mouth was shut, when the jaws were separated a space would be left beneath it through which the water and the minute particles of food would escape together. But, instead of this, the long, slender, brush-like ends of the whalebone-blades, when the mouth is closed, fold back, the front ones passing below the hinder ones in a channel lying between the tongue and the bone of the lower jaw. When the mouth is opened their elasticity causes them to straighten out like a bow that is unbent, so that, at whatever distance the jaws are separated, the strainer remains in perfect action, filling the whole of the interval. The mechanical perfection of the arrangement is completed by the great development of the lower lip, which rises stiffly above the jawbone, and prevents the long, slender, flexible ends of the baleen being carried outward by the rush of water from the mouth.
Few points of the structure of whales offer so great a departure from the ordinary mammalian type as the limbs. The fore-limbs are reduced to the condition of simple paddles or oars, variously shaped, but always flattened and more or less oval in outline. They are freely movable at the shoulder-joint, where the humerus or upper-arm bone articulates with the shoulder-blade in the usual manner, but beyond this point, except a slight flexibility and elasticity, there is no motion between the different segments. The bones are all there, corresponding in number and general relations with those of the human or any other mammalian arm, but they are flattened out, and their contiguous ends, instead of presenting hinge-like joints, come in contact by flat surfaces, united together by strong ligamentous bands, and all wrapped up in an undivided covering of skin, which allows externally of no sign of the separate and many-jointed fingers seen in the skeleton.
The changes that have taken place in the hind-limbs are even more remarkable. In all known Cetacea (unless Platanista be really an exception) a pair of slender bones are found suspended a short distance below the vertebral column, but not attached to it, about the part where the body and the tail join. In museum skeletons these bones are often not seen, as, unless special care has been taken in the preparation, they are apt to get lost. They are, however, of much importance and interest, as their relations to surrounding parts show that they are the rudimentary representatives of the pelvic or hip bones, which in other mammals play such an important part in connecting 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 of whales equaling in size those that are now to be met with in the ocean. The size of whales, as of all other things whose most striking attribute is magnitude, has been greatly exaggerated; but, when reduced to the limits of sober fact, the Greenland right whale of fifty feet long, the sperm whale of sixty, and the great northern rorqual (Balœnoptera Sibbaldii) of eighty, exceed all other organic structures known, past or present. Instead of living in an age of degeneracy of physical growth, we are in an age of giants, but it may be at the end of that age. For countless ages impulses from within and the forces of circumstances from without have been gradually shaping the whales into their present wonderful form and gigantic size, but the very perfection of their structure and their magnitude combined, the rich supply of oil protecting their internal parts from cold, the beautiful apparatus of whalebone by which their nutrition is provided for, have been fatal gifts, which, under the sudden revolution produced on the surface of the globe by the development of the wants and arts of civilized man, can not but lead in a few years to their extinction.
Let us return to the question with which we started, "What was the probable origin of whales?" The evidence is absolutely conclusive that they were not originally aquatic in habit, but are derived from terrestrial mammals of fairly high organization, belonging to the placental division of the class—animals in which a hairy covering was developed, and with sense-organs, especially that of smell, adapted for living on land; animals, moreover, with four completely developed pairs of limbs on the type of the higher vertebrata, and not of that of fishes.
One of the methods by which a land mammal may have been changed into an aquatic one is clearly shown in the stages which still survive among the carnivora. The seals are obviously modifications of the land carnivora, the Otaria, or sea-lions and sea-bears, being curiously intermediate. Many naturalists have been tempted to think that the whales represent a still further stage of the same kind of modification. But there is to my mind a fatal objection to this view. The seal, of course, has much in common with the whale, inasmuch as it is a mammal adapted for an aquatic life, but it has been converted to its general fish-like form by the peculiar development of its hind-limbs into instruments of propulsion through the water; for, though the thighs and legs are small, the feet are large and are the special organs of locomotion in the water, the tail being quite rudimentary. In the whales the hind-limbs are aborted and the tail developed into a powerful swimming organ. Now, it is very difficult to suppose that, when the hind-limbs had once become so well adapted to a function so essential to the welfare of the animal as that of swimming, they could ever have become reduced and their action transferred to the tail. It is far more reasonable to suppose that whales were derived from animals with large tails, which were used in swimming, eventually with such effect that the hind-limbs became no longer necessary, and so gradually disappeared. The powerful tail, with lateral cutaneous flanges, of an American species of otter (Pteronura sandbachii), or the still more familiar tail of the beaver, may give some idea of this member in the primitive Cetacea.
As pointed out long ago by Hunter, there are numerous points in the structure of the visceral organs of the Cetacea far more resembling those of the Ungulata than the Carnivora. These are the complex stomach, simple liver, respiratory organs, and especially the reproductive organs and structures relating to the development of the young. I can not help thinking that some insight has been shown in the common names attached to one of the most familiar of Cetaceans by those whose opportunities of knowing its nature have been greatest—"sea-hog," "sea-pig," or "herring-hog," of our fishermen, Meerschwein of the Germans, corrupted into the French "marsouin," and also "porcpoisson," shortened into "porpoise." A difficulty that might be suggested in the derivation of the Cetacea from the Ungulata, arising from the latter being at the present day mainly vegetable-feeders, is not great, as the primitive ungulates were probably omnivorous, as their least modified descendants, the pigs, are still; and the aquatic branch might easily have gradually become more and more piscivorous, as we know, from the structure of their bones and teeth, the purely terrestrial members have become by degrees more exclusively graminivorous.
One other consideration may remove some of the difficulties that may arise in contemplating the transition of land mammals into whales. The Gangetic dolphin (Platanista) and the somewhat related Inia of South America, which retain several rather generalized mammalian characters, and are related to some of the earliest known European Miocene forms, are both to the present day exclusively fluviatile, being found in the rivers they inhabit almost up to their very sources, more than a thousand miles from the sea. May this not point to the freshwater origin of the whole group, and thus account for their otherwise inexplicable absence from the Cretaceous seas?
- Abridged from a lecture delivered at the Royal Institution, London, May 25, 1883.