ment sounds, there are a great many birds that do not fly: and it is among these terrestrial or swimming kinds that we must look for the nearest modern approaches to the primitive bird type. From the very beginning, birds had to endure the fierce competition of the mammals, which had been developed at a slightly earlier period; and they have for the most part taken almost entirely to the air, where alone they possess a distinct superiority over their mammalian compeers. There are certain spots, however, where mammals have been unable to penetrate, as in oceanic islands; and there are certain other spots which were insulated for a long period from the great continents, so that they possessed none of the higher classes of mammals, as in the case of Australia, South America, New Zealand, and South Africa. In these districts, terrestrial birds had a chance which they had not in the great circumpolar land tract, now divided into two portions, North America on the west, and Asia and Europe on the east. It is in Australia and the southern extremities of America and Africa, therefore, that we must look for the most antiquated forms of birds still surviving in the world at the present day.
The decadent and now almost extinct order of struthious birds, to which ostriches and cassowaries belong, supplies us with the best examples of such antique forms. These birds are all distinguished from every other known species, except the transitional Solenhofen creature and a few other old types, by the fact that they have no keel to the flat breast-bone—a peculiarity which at once marks them out as not adapted for flight. Every one whose anatomical studies have been carried on as far as the carving of a chicken or a pheasant for dinner knows that the two halves of the breast are divided by a sharp keel or edge protruding from the breast-bone; but in the ostrich and their allies such a keel is wanting, and the breast-bone is rounded and blunt. At one time these flat-chested birds were widely distributed over the whole world; for they are found in fossil forms from China to Peru; but, as the mammalian race increased and multiplied and replenished the earth, only the best adapted keeled birds were able to hold their own against these four-legged competitors in the great continents. Thus the gigantic ostriches of the Isle of Sheppey and the great divers of the Western States died slowly out, leaving all their modern kindred to inhabit the less progressive southern hemisphere alone. Even there, the monstrous æpyornis, a huge, stalking, wingless bird, disappeared from Madagascar in the tertiary age, while the great moa of New Zealand, after living down to almost historical times, fell a victim at last to that very aggressive and hungry mammal, the Maori himself. This almost reduces the existing struthious types to three small and scattered colonies, in Australasia, South Africa, and South America respectively, though there are still probably a few ostriches left in some remote parts of the Asiatic Continent.
The Australian ostrich kind are in many respects the most archaic
