xm PHYLUM CHORDATA 429 



X 



By this time the remainder of the yolk-sac has been drawn into 

 the coelome, and the ventral body-walls have closed round it. 

 On the shell being broken respiratory movements begin, the 

 aperture is enlarged, and the young bird is hatched and begins a 

 free life. 



In the Ratitae, Anseres, Gallinse, and some other Birds the young 

 when hatched are clothed with a complete covering of down or of 

 feathers, and are able from the first to run about and feed them- 

 selves ; such Birds are called Prcecoces or Nidifugce. In the higher 

 types, such as the Rapacious Birds, Pigeons, and Passeres, the 

 young are at first either quite naked, blind and helpless, or covered 

 with mere patches of soft down, so that they require to be fed and 

 kept warm by the parents ; these forms are called Altrices or 

 Nidicolce. In many Sea-birds, such as Petrels, Gulls, and Penguins, 

 the young have a complete covering of woolly down, but remain 

 in the nest for a prolonged period, sometimes until the full size 

 is attained. 



Distribution. The Ratitse furnish an interesting case of dis- 

 continuous distribution. Struthio occurs in Africa and South- 

 western Asia, Rhea in South America, Dromaeus in Australia, 

 Casuarius in Australia, New Guinea, and some of the other Austro- 

 Malayan islands, and Apteryx in New Zealand. Thus, taking 

 recent forms only, each of the great Southern land-masses contains 

 one order of Ratitse not found elsewhere ; the Struthiones are 

 Ethiopian, but extend also into the adjacent part of the Palsearctic 

 region, the Rhese Neotropical, and the Megistanes Australasian. 

 ^Epyornis, the affinities of which appear to be with the Megis- 

 tanes, occurs only in Madagascar, where it has become extinct 

 within geologically speaking comparatively recent times. When 

 we take the scattered distribution of the above-mentioned Ratitse 

 into consideration, one of the most remarkable facts in distribution 

 is the occurrence, in the limited area of New Zealand, of no fewer 

 than six genera and between twenty and thirty species of Dinor- 

 nithidse or Moas, some of which became extinct so short a time 

 ago that their skin, flesh, feathers, dung, and egg-shells are pre- 

 served. 



Among the Carinatse the Penguins are exclusively southern, 

 occurring only in the South Temperate and Arctic Oceans. They 

 may be said to be represented in the Northern Hemisphere by the 

 Puffins and Auks, one of which, the Great Auk or Gare-fowl (Alca 

 impennis), was actually impennate, its wings being converted, as in 

 the Penguins, into paddles. The Crypturi (Tinamous) are exclu- 

 sively Neo-tropical, the Humming-birds American, the Birds of 

 Paradise and Bower-birds Australian and Austro-Malayan. Amongst 

 negative facts, the Psittaci or Parrots are characteristically absent 

 in the Palaearctic and most of the Nearctic region, the Finches in 



