Thus, that great group which embraces man and beasts, and which group ranks as a "class"--the _class_ Mammalia--comprises (as we have now seen) a number of subordinate groups termed "orders," the orders being made up of families, and these again of genera. It would be impossible as yet (when hardly any anatomical facts have been even referred to) to give the characters of the class _Mammalia_. It must at present suffice to point out that, in addition to mammary glands, the creatures have hot blood, and the body bears more or less hair--at least at some time of life. We may now pass to the next class, that of birds--the class _Aves_. In spite of the great multitude of kinds which ornithologists enumerate--upwards of ten thousand species--there is very much less diversity of form amongst birds than there is amongst beasts. Starting in the present class as in the preceding one from the most familiar kinds, we may begin with the domestic fowl. This is one of an "order" to which belong the peacock, all pheasants and tragopans (three forms which have their home in Central and Southern Asia), also the Guinea fowls (African forms), and the turkeys and curassows, which are American representatives of the order. Besides these may be mentioned partridges, grouse, black-cock, the capercalzie and quails, and, lastly, the megapodius or bush-turkey of Australia. This last is the only bird which hatches its eggs by artificial heat, depositing them in a mound of earth and decaying vegetable matter, wherein they are hatched fully-fledged, so that they can fly away immediately on leaving the egg. All the birds yet mentioned are called gallinaceous birds, or _Gallinæ_, and sometimes _Rasores_ or "Scratchers." More or less allied to them are the doves and pigeons, which form the order _Columbæ_, in which the curious ground-pigeon _Didunculus_ is included--a form which presents an interesting resemblance to the celebrated and extinct dodo of Mauritius, long known only by certain pictures, and a foot and head preserved, one in the British Museum, and the other in the Ashmolean Museum of Oxford. Our sparrows, robins, and all our song birds are members of an exceedingly numerous "order" "_Paseres_." In it are included the crows (with those gaily-decorated crows, the Birds of Paradise, found only in New Guinea and the Moluccas), the bower birds and the lyre bird of Australia; the flycatchers, the pittas (or ground thrushes), the water-ouzel, the weaver birds, the wrens, the tits, the creepers, the honey-eaters, those African gems, the sun birds, and also the swallows. To another order--the order _Macrochires_--belong those most beautiful of all birds, the humming birds, found only in America, and long thought to be allied with the really very different sun birds just mentioned. With these may be associated the swifts (which have such marvellous powers of flight) and the wide-gaped goat-suckers or nightjars.