THE ANCESTRY OF BIRDS. 613 



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 pene- 

 trate, 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, there- 

 fore, that we must look for the most antiquated forms of Jbirds 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 ex- 

 amples 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 kin- 

 dred to inhabit the less progressive southern hemisphere alone. Even 

 there, the monstrous sepyornis, a huge, stalking, wingless bird, disap- 

 peared 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 



