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

THE FIRST GREAT PHYLUM: VERTEBRATES

at present it will be more profitable for us to examine one by one the various classes of Vertebrates and so get an idea of what they are like than to plunge into anatomical details. Nevertheless, there is a simple definition ; the Vertebrates are the athletes of the Animal Kingdom. ‘They have betterdeveloped muscles, and better devices for supplying those muscles with oxygen than any other creatures. The mammals on land, the birds in the air, the fish in the water—these include in their ranks the largest, strongest, and swiftest of creatures, and those with the greatest powers of endurance. That is why their one phylum thrusts itself upon our attention in front of all the other phyla of the Animal Kingdom. Theirs is the leading pattern ; they include all the high-grade makes in the world of life.

§ 3.

The Classes of the Vertebrate Phylum

At the head of any survey of Vertebrate forms must come the mammals and birds. They have dominated this earth for many millions, perhaps for scores of millions of years, and are manifestly the best adapted creatures to the present climatic variety and uncertainties of our planet. But they have not always ruled the terrestrial scene. In the past, stupendous ages passed in which neither bird flew nor mammal ran, and beings less adapted to an active life in a fluctuating temperature prevailed. There was an age when the reptiles ruled, before Nature it would seem had invented the hair or feathers, the double circulation and the blood at a constant temperature, which account for the active presence of the bird and mammal all over the globe and all round the year.

§ 3a Mammals

The class Mammalia includes most of the animals with which we are familiarourselves and dogs, cats and horses, pigs and mice. Mammals are warm-blooded animals, a distinction which they share with the birds, and to prevent loss of heat they are more or less completely clothed with hair. This hair, we may note, is one of the distinguishing features of mammals. Moreover they are peculiar in the treatment of their young. For the most part they bring forth their young alive instead of laying eggs after the fashion of most creatures, and they are unique

in that their females have special milkproducing glands with which to suckle their offspring. ‘These are the most characteristic points, but there are others. The muscular diaphragm which separates chest from belly is a characteristic feature of the mammals ; in other vertebrates the heart and lungs and bowels all writhe and jostle in one large body-cavity. Mammals have better-developed brains, and consequently better minds than any other creatures. And while the lower jaw of all other vetebrates consists of several bones joined together, the mammals have a lower jaw of one single bone. This fact, which may seem unimportant to the reader, is of very great value in tracing the evolution of the group, because in many extinct species, and particularly early mammalian species, only the lower jaws have been found, owing to the ease with which lower jaws drop off floating carcasses, to be embedded and preserved in river mud.

Now there are three grades of Mammals, differing in the manner in which they treat their young. The first grade, the Monotremes, is a very small one ; it includes two curious creatures, the spiny ant-eater and the duck-bill or water-mole, which show a number of striking resemblances to the reptiles. They stand markedly apart from the rest of the class. They differ from the other members of the class in that they lay eggs instead of bearing young; nevertheless, when the young ones hatch, they are suckled by the mother. In their egg-laying habits they are reptile-like, and they show other reptilian features in their skeletons, in the structure of the brain, and in not being able to keep their temperature as constant as that of other mammals. The duck-bill is found only in Australia and Tasmania ; it lives in streams, feeding on insects, snails, small bivalve molluscs and the like, and it makes a burrow in the bank where it lays its eggs. The spiny ant-eaters, of which there are two genera and several species, are found in Australia, Tasmania and New Guinea ; the females carry their eggs in a large pocket in the skin of the belly until they hatch. These creatures are fascinating from the evolutionary point of view; they are survivors of a time when some of the reptiles were turning themselves into mammals by acquiring warm blood and hair.

The remaining mammals fall into two parallel sections, of which one is very much larger than the other. ‘The first and smaller section is the Marsupials, or pouched mammals. These animals are intermediate between the egg-laying monotremes and the

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