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

THE FIRST GREAT PHYLUM: VERTEBRATES

swallow stones and use them in the gizzard (a special part of the stomach) to grind up their food, and there are birds that eat meat and have no need of such a device; there are various shapes of beak and neck, and a tremendous range of coloration. But compared, for example, with the mammals the variation of structure found among birds is slight. Indeed, the only birds which show really profound departures are those exceptional ones which do not fly through air—the penguins, which fly in a medium having very different physical properties, and the completely flightless kiwis, ostriches, rheas, emus, and cassowaries, in which the wings are small and often useless, and in which, since there is no need of powerful muscles to move them, the keel on the breast-bone is small or absent.

§ 3c The More Ancient Class of Reptiles

The three greatest classes of vertebrates to-day are the mammals, birds, and fishes. It is as if these three classes had divided the world between them, the mammals taking the land, the birds the air, and the fishes the water. The fishes rarely leave their proper element, but each of the other two groups has sent a few representativesmissionaries and prospectors as it were—

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into the allotted territory of the other ; the class of mammals, for example, has sent the bats into the air and the whales into the

sea. Nevertheless, in the main, each of the three classes rules in its own element. The remaining classes are now less dominant, and of minor importance ; but among them the reptiles, at least, have a great history. At one time they ruled both land and sea, and birds and mammals were not.

In a number of points the reptiles are like mammals or birds—their embryos, for example, are surrounded in the egg by an elaborate system of membranes like the membranes of a chick in the egg, or of a developing mammal in the uterus, and they breathe throughout their life-histories by means of lungs—but they are cold-blooded, that is to say the temperature of their blood has to be that of their surroundings, and they have scales instead of the hair of mammals or the feathers of birds.

There are four orders of reptiles living. The first includes the crocodiles, alligators and gavials—predatory animals living in tropical rivers, some snapping the heads of mammals that come to drink and holding them under water until they drown, others taking fish and the like. ‘The second consists of the tortoises, turtles and terrapins, landliving and aquatic, their bodies encased in a hard, protective shell into which in some,

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