The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

utterly surrendered their land-living habits ; even seals come ashore to breed. But the whales never set foot on land. Indeed, they have no feet ; their front limbs are small, like the front pair of fins in a fish, and are used as balancing “ flippers’? ; their hind limbs are represented by mere splints of bone, embedded in the body-wall, which are not used at all; their tails have broad horizontal flukes which are used, like the tail of a fish, to drive the body along. Moreover, the group includes the largest known animals, for the blue whale weighs over a hundred tons. The smallest mammal of all, we may note, is a common insectivore, the pigmy shrew, which is less than a millionth of that weight. Even the smallest Cetaceans, when fully adult, are three feet long. As regards diet, there are two kinds of whales. Some, the toothed whales, eat large prey (such as giant cuttle-fish). The common porpoise, which chases and devours fish, is of this kind. The others, the whalebone whales, have a sieve of whalebone round their mouth in the place of teeth and they feed on the tiny creatures that swarm in myriads at the surface of the sea; they take a large mouthful of sea and squirt it out again through their internal moustache of whalebone, and then lick off the masses of tiny creatures that have stuck on the sieve. Most whales are exclusively marine, but there are one or two species which venture into rivers—the common porpoise, for example, has ascended the Seine to Paris. Moreover, as the only finned mammals, they occupy a unique position in human economics ; to the Catholic Church they have always been fish and therefore eatable in Lent. Hence the name of the porpoise, which is a corruption of Porkpisce, or pork fish.

All the mammalian cohorts include a great number of orders, families and genera now extinct, and in many cases these extinct forms are interesting because they form links and bridges between groups of which the surviving members are in sharp contrast. But the consideration of ‘these extinct and linking forms we will reserve for a later Book.

§ 3b Birds

Birds appeared in the world in the same middle period of time as the mammals, and became abundant as mammals became abundant. They are a parallel class of heat-retaining, high-efficiency animals.

Birds obviously are heavier-than-air flying machines. There are indeed one or two

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THE SCIENCE OF LIFE

GHAPTER 1

birds that cannot fly—the ostrich, for example, or the kiwi—and the penguin, instead of flying in air, flies in water, but the great majority of them are able to take to the air,and the fact is reflected in every bone, every muscle, every internal organ. Moreover, they are the only class of animalsexcept the insects—the majority of whose members are winged. They include the largest and heaviest flying animals, and their champions can fly higher, more swiftly and for a longer time than any other unaided living creature.

Birds are characterized by the possession of feathers, just as are mammals by hair ; feathers long and used for propulsion and steering on the wings and tail, short and forming an efficient garment on the rest of the body in order to reduce the amount of heat lost to the air that streams past during flight. Their arms are wings, and their breast-bone bears an enormous keel in front to which the muscles which flap the wings downwards are attached. Being wings, the arms cannot be used to support the body, and when the bird is not flying its whole weight is poised on the legs; the hip-girdle is therefore very solidly joined to the backbone, the feet are large, and the legs are struts, firm but light. Moreover, except for a few extinct forms, birds have no teeth, and their jaws are ensheathed by horny beaks made of the same stuff as the dead outer layer of our skins, and variously moulded according to the feeding habits of the bird. In their internal anatomy there are several devices to assist the bird in performing vigorous and sustained exercise which necessitates a copious oxygen supply to the muscles. Their hearts are larger in proportion to the size of the body than those of mammals, and their lungs communicate with a unique system of air-spaces which make the oxygen supply of the body more efficient—some running among the viscera and some into the bones, which, being hollow, combine the maximum of rigidity with the minimum of weight. :

The mechanical problems of propelling a heavy body through air are so considerable, and involve specialization of structure in so many parts of the body, that bird organization is to a large extent stereotyped. There is little room for variation. To be sure, there are wading, swimming, perching, and running birds, each with a different kind of leg and foot; there are soaring, hovering, and fluttering birds, and clean, swift fliers, each with its own shape of wing and tail; there are vegetable-feeders that