IV. ADAPTATION AS SHOWN BY THE CETACEA
There are many indisputable evidences that whales once lived upon the land and walked upon four legs like ordinary quadrupeds, yet how remarkably different from any land mammal is their present form!
We see that almost all aquatic creatures have torpedo-shaped bodies, which offer the minimum of resistance to their passage through the water. Thus as the whales gradually changed from a terrestrial to an aquatic life their bodies assumed the elongated form essential for successful existence in a liquid medium.
Accompanying this change of bodily shape was the elimination of all unnecessary structures which offered resistance, and the whale’s smooth, soft, hairless skin was one of the results. But the hair of a land mammal acts as a non-conductor, preventing the heat of the blood from being absorbed by the air, and as the whale’s body became naked it was necessary to blanket it with some other protective covering; thus the layer of fat or blubber developed between the skin and the flesh. Fish and amphibians do not need a warm covering because their blood is cold and changes with the temperature of the medium in which they live.
Besides giving warmth to a land mammal, hair acts as a protection for its tender skin; but since a whale lives in the water, where bruises or abrasions are unlikely, such protection is unnecessary. With the loss of hair the sweat and oil glands which are present in the skins of land mammals finally disappeared.
When any creature becomes aquatic it must necessarily develop means for progression through the water, and thus the caudal portion of the whale’s body by degrees expanded into the wide, flat, boneless tail, or flukes. But instead of being vertical to the axis of the body like the tail of a fish, the whale’s flukes are horizontal, obviously to give the animal greater facility in rising to the surface to breathe.
With the development of the flukes there came a change in the whale’s fore-limbs, which were flattened and covered with connective tissue and blubber. The excellent paddles thus formed, while probably of little use in forward motion, assist in rapid turning and act as balancing organs to keep the animal upright in the water. In some species an adipose dorsal fin has also developed as a further balancing aid.
During the development of the flippers and flukes the hind-limbs, which were no longer of use to the whale, became small and weak, sunk into the blubber and finally disappeared altogether, the greatly modified pelvic elements and nodules of bone or cartilage representing the femur alone remaining.
The heads of most cetaceans are long and pointed, acting as a “cut-water,” but one of the most remarkable aquatic adaptations is the position of the nostrils, or blowholes, which open upon the very summit of the head, in either a single or double aperture, instead of at the end of the snout. The cause of this migration of the nostrils is obvious, for in this position the blowholes first appear at the surface and the whale can begin to breathe while the rest of its body is yet under water.
In all cetaceans the facial portion of the skull is greatly elongated, and especially in the Mystacoceti the mouth is exceedingly large to accommodate the baleen, which hangs in two parallel rows from the upper jaw. Probably no mammalian adaptation for the securing of food is more remarkable than the whale’s baleen. It is almost unbelievable that an animal which once had teeth could, as its food changed, replace them by a complicated straining apparatus such as the whalebone. The baleen is an epidermal growth and is in reality merely an exaggeration of the transverse ridges present in the mouths of land mammals.
We know that the Mystacoceti at one time had teeth, for in fœtal whales two sets of minute teeth are present under the skin, corresponding to the “milk” and “permanent” dentition of ordinary mammals, but these disappear before the baleen begins to develop.
Another interesting feeding adaptation is present in the throat of the whale. The nostrils, instead of opening into the back of the mouth, as in land mammals, are directly connected with the lungs by a prolongation of the “windpipe” called the epiglottis, which entirely shuts off the whale’s breathing passage from the mouth. Thus the animal can swallow its food beneath the surface without danger of strangulation through getting water into its lungs.
When whales lived upon the land external ears were necessary, but as they became completely aquatic such “sound collectors” were not only of no more use but highly undesirable, because, like the useless hind-limbs, they offered additional resistance to the water; therefore the external ears were lost, but their muscles still remain about the minute ear-orifices of the present-day Cetacea.
The internal modifications which the whales underwent as they assumed an aquatic existence are fully as remarkable as the external changes. In the section on osteology it has been explained how, in living cetaceans, the entire skeleton is loosely articulated so that great flexibility and freedom of movement is given to the body, how the neck is shortened and the vertebræ have become thin and closely packed together to support the large head, and how the breast bone is reduced and the ribs so loosely articulated to the vertebral column that the huge lungs have full power of expansion. All these are necessary modifications of the mammalian skeleton which have been caused by the change from a terrestrial to an aquatic existence.
The lungs of the Cetacea are unlobulated and of extraordinary size; the diaphragm, the muscular partition which separates the thoracic from the abdominal cavity, is oblique, and the brain greatly convoluted and of a high type; the brain is especially notable for the loss of the olfactory, or smelling portions, which are of no use to an aquatic mammal.
Thus it is apparent in a review of only the most obvious changes what a wonderful example of adaptation to environment is furnished by the Cetacea.