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 the inner
plicated surface of 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 more deeply in the
head, but 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 favourable 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.
The perception of odorous substances diffused in the air, upon which
many land mammals depend so much for obtaining their food, or for
protection from danger, can be of little importance to them. 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 neither group is there the slightest trace of
the specially aquatic olfactory organ of fishes. Its complete absence
and the presence of vestiges of the aerial organ of land mammals in the
Mystacocetes are the clearest possible indications of the origin of the
Cetacea 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 do not seem to have permitted the
acquisition of anything analogous to the specially aquatic smelling
apparatus of fishes, the result being that whales are practically
deprived of whatever advantage this sense may be to other animals.
It is characteristic of the greater number of mammalia to have their
jaws furnished with teeth having a definite structure and mode of
development. In all the most typical forms these teeth are limited in
number, not exceeding eleven on each side of each jaw, or forty-four in
all, and are differentiated in shape in different parts of the series,
being more simple in front, broader and more complex behind. Such a
dentition is described as “heterodont.” In most cases also there are two
distinct sets of teeth during the lifetime of the animal, constituting a
condition technically called “diphyodont.”
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. In the first place, at all events in existing species,
they are never truly heterodont, all the teeth of the series resembling
each other more or less, or belonging to the condition called
“homodont,” and not obeying the usual numerical rule, often falling
short of, but in many cases greatly exceeding it. The most typical
Odontocetes, or toothed whales, have a large number of similar, simple,
conical, recurved, pointed teeth, alike on both sides and in the upper
and under jaws, admirably adapted for catching slippery, living prey,
such as fish, which are swallowed whole without mastication. In one
genus (_Pontoporia_) there may be as many as sixty of such teeth on
each side of each jaw, making 240 in all. The more usual number is from
twenty to thirty. These teeth are never changed, being “monophyodont,”
and they are, moreover, less firmly implanted in the jaws than in land
mammals, having never more than one root, which is set in an alveolar
socket, generally wide and loosely fitting, though perfectly sufficient
for the simple purpose which the teeth have to serve.
Most singular modifications of this condition of dentition are met with
in different genera of toothed whales, chiefly the result of
suppression--sometimes of suppression of the greater number, combined
with excessive development of a single pair. In one large group, the
Ziphioids, although minute rudimentary teeth are occasionally found in
young individuals, and sometimes throughout life, in both jaws, in the
adults the upper teeth are usually entirely absent, and those of the
lower jaw reduced to two, which may be very large and projecting like
tusks from the mouth, as in _Mesoplodon_, or minute and entirely
concealed beneath the gums, as in _Hyperoodon_,--an animal which is for
all practical purposes toothless, yet in which a pair of perfectly
formed though buried teeth remain throughout life, wonderful examples of
the persistence of rudimentary and to all appearance absolutely useless
organs. Among the _Delphinidæ_ similar cases are met with. In the genus
_Grampus_ the teeth are entirely absent in the upper, and few and early
deciduous in the lower jaw. But the narwhal exceeds all other Cetaceans,
perhaps all other vertebrated animals, in the specialisation of its
dentition. Besides some irregular rudimentary teeth found in the young
state, the entire dentition is reduced to a single pair, which lie
horizontally in the upper jaw, and both of which in the female remain
permanently concealed within the bone, so that this sex is practically
toothless, while in the male the right tooth usually remains similarly
concealed and abortive, and the left is immensely developed, attaining a
length equal to more than half that of the entire animal, projecting
horizontally from the head in the form of a cylindrical or slightly
tapering pointed tusk, with the surface marked by spiral grooves and
ridges.
The meaning and utility of some of these strange modifications 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 specialised forms as
being one derived from the more generalised type, in which the teeth
were numerous and equal.
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 St. Hilaire, and since amply
confirmed by Cuvier, Eschricht, Julin, and others, that in the fœtal
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 fœtal 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 that
characteristic of ordinary mammalian teeth, and it has also been
observed that those at the posterior part of the series are larger, and
have a bilobed form of crown, while those in front are simple and
conical, a fact of considerable interest in connection with speculations
as to the history of the group.
It is not until after the disappearance of these teeth that the baleen,
or whalebone, makes its appearance. This remarkable structure, though,
as will be presently shown, only a modification of a part existing in
all mammals, is, in its specially developed condition as baleen,
peculiar to one group of whales. It is therefore perfectly in accord
with what might have been expected, that it is comparatively late in
making its appearance. Characters that are common to a large number of
species appear early--those that are special to a few, at a late period,
alike both in the history of the race and of the individual.
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 downwards. The outer edge of the blade is hard and
smooth, but the inner edge and apex fray out into long, bristly fibres,
so that the roof of the whale’s mouth looks as if covered with hair, as
described by Aristotle. The blades are longest near the middle of the
series, and gradually diminish towards 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 maxillæ, and which sends out
lamellar processes, one of which penetrates the base of each blade.
Moreover, the free edge of each 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 fibres of which the blade is mainly
composed. A transverse section of fresh whalebone shows that it is made
up of numbers of these soft vascular papillæ, circular in outline, each
surrounded by concentrically arranged epidermic cells, the whole bound
together by other epidermic cells, which constitute the smooth cortical
(so-called “enamel”) surface of the blade, and which, disintegrating at
the free edge, allows the individual fibres to become loose and to
assume the hair-like appearance spoken of before. These fibres differ
from hairs in not being formed in depressed follicles in the enderon,
but rather resemble those of which the horn of the rhinoceros is
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
whalebone 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 molluscs, 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
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
whalebone 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, as I have mentioned, 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 coat 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 backwards; 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. In oxen, and especially in the giraffe, they form distinct
laminæ, and their free edges develop a row of pointed papillæ, giving
them a pectinated appearance. Their epithelium is thick, hard, and
white, though not horny. Although the interval between the structure of
the ridges in the giraffe’s palate and the most rudimentary form of
baleen at present known is great, there is no difficulty in seeing that
the latter is essentially a modification of the former, just as the hoof
of the horse, with its basis of highly developed vascular laminæ and
papillæ, and the resultant complex arrangement of the epidermic cells,
is a modification of the simple nail or claw of other mammals, or as the
horn of the rhinoceros is only a modification of the ordinary derm and
epiderm covering the animal’s body differentiated by a local exuberance
of growth.
Though the early stages by which whalebone has been modified from more
simple palate structures are entirely lost to our sight, probably for
ever, 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. These variations are worth dwelling upon for a few
minutes, as they illustrate in an excellent manner the gradual
modifications that may take place in an organ, evidently in adaptation
to particular requirements, the causation of which can be perfectly
explained upon Darwin’s principle of natural selection.
In the rorquals or fin-whales (genus _Balænoptera_, Fig. 12, p. 198),
found in almost all seas, and so well known off our own coasts, the
largest blades in an animal 70 feet long do not exceed 2 feet in length,
including their hairy terminations; they are in most species of a pale
horn colour, and their structure is coarse and inelastic, separating
into thick, stiff fibres, 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 is 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 in the Atlantic and
southern right whales. Before describing the extreme modifications in
the direction of complexity, I may mention, to show the range at present
presented in the development of baleen, that there has lately been
discovered in the North Pacific a species called by the whalers the
Californian grey whale (_Rachianectes glaucus_), which shows the
opposite extreme of simplicity. The animal is from 30 to 40 feet in
length; the baleen blades are only 182 on each side (according to
Scammon) and far apart, very short (the longest being from 14 to 16
inches in length), light brown or nearly white in colour, and still more
coarse in grain and inelastic than that of the rorquals. The food of
these whales is not yet known with certainty. They have been seen
apparently seeking for it along soft bottoms of the sea, and fuci and
mussels have been found in their stomachs.
In the Greenland right whale of the circumpolar seas, the bowhead of the
American whalers (_Balæna mysticetus_, Fig. 9, p. 195), 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 backwards, 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 350 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 colour, 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.
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 the nature of 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 provides 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 outwards by the
rush of water from the mouth, when its cavity is being diminished by the
closure of the jaws and raising of the tongue. The interest and
admiration excited by the contemplation of such a beautifully adjusted
piece of mechanism is certainly heightened by the knowledge that it has
been brought about by the gradual adaptation and perfection of
structures common to the whole class of animals to which the whale
belongs.
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.
Up to the year 1865 it was generally thought that there was nothing to
be found between this bony framework and the covering skin, with its
inner layer of blubber, except dense fibrous tissue, with blood-vessels
and nerves sufficient to maintain its vitality. Dissecting a large
rorqual, 67 feet in length, upon the beach of Pevensey Bay in that year,
I was surprised to find lying upon the bones of the forearm
well-developed muscles, the red fibres of which reached nearly to the
lower end of these bones, ending in strong tendons, passing to, and
radiating out on, the palmar surface of the hand. Circumstances then
prevented me following out the details of their arrangement and
distribution, but not long afterwards Professor Struthers, of Aberdeen,
had an opportunity of carefully dissecting the fore-limb of another
whale of the same species, and he has recorded and figured his
observations in the _Journal of Anatomy_ for November 1871. He found on
the internal or palmar aspect of the limb three distinct muscles
corresponding in attachments to the flexor carpi ulnaris, the flexor
profundus digitorum, and the flexor longus pollicis of man, and on the
opposite side but one, the extensor communis digitorum.[38] Large as
these muscles actually are, yet, compared with the size of the animal,
they cannot but be regarded as rudimentary, and being attached to bones
without regular joints and firmly held together by unyielding tissues,
their functions must be reduced almost to nothing. But rudimentary as
the muscles of the fin-whales are, lower stages of degradation of the
same structures are found in other members of the group. In some they
are indeed present in form, but their muscular structure is gone, and
they are reduced in most of the toothed whales to mere fibrous bands,
scarcely distinguishable from the surrounding tissue which connects the
inner surface of the skin with the bone. It is impossible to contemplate
these structures without having the conviction forced home that here are
the remains of parts once of use to their possessor, now, owing to the
complete change of purpose and mode of action of the limb, reduced to a
condition of atrophy verging on complete disappearance.