132 Degeneration in the Ostrich 



wings, the disappearance of a sternal keel and of elements from the 

 shoulder girdle, and a reduction in the number of toes. As illustrations 

 of one degenerative phase or another we need only recall the well-known 

 retrogressive features associated with the recently extinct moas, and 

 the still surviving ostrich, kiwi, rhea and various cassowaries. 



Among the Ratitae the African ostrich, Struthio, affords distinctive 

 evidence of some degenerative power at work in the small size of the 

 wings and the unique reduction of the toes to two, as well as in the 

 absence of plumage in various directions. A close study of the bird 

 reveals other losses in less manifest directions, diverse stages having 

 been reached in different individuals of the race. 



It would therefore be expected that both on its own account and as 

 a member of a retrogressive sub-class the ostrich would be peculiarly 

 favourable for a study of the manner in which degeneration proceeds, as 

 possibly throwing some light upon the nature of variation and the 

 methods of evolution generally. An abundance of material is available 

 among the thousands of domesticated birds on the ostrich farms of South 

 Africa, supplemented by a recent importation of North African birds for 

 crossing with the southern. From these it becomes possible to compare 

 the course of degeneration over practically the whole continent, while 

 embryological material enables the various ontogenetic phases to be 

 followed from the earliest stage onwards. The results resolve themselves 

 into a series of more or less separate studies in degeneration, though 

 with certain considerations common to all. Special attention is given 

 to the more manifest features associated with the plumage, the fore- 

 wing and the toes, though these by no means exhaust the directions 

 along which loss has taken, and is taking place. The facts disclosed will 

 also be seen to have an important bearing upon the ostrich as a domesti- 

 cated bird, constituting as it does an important farming industry for 

 South Africa. 



Though the plumage is the only part of the ostrich of importance 

 from a farming and industrial point of view, we are probably justified in 

 remarking that it is more scantily provided with feathers than any 

 other living bird. The under surface of the wings (Fig. 1), the lateral 

 part of the body from the axillary region to the tail, as well as the entire 

 surface of the legs, are practically devoid of feathers in the adult (PI. VI, 

 fig. 4); also a large apterium occurs over the sternal callosity in front, and 

 another over the pubic thickening behind ; in addition, the undercover- 

 ingof down has all but disappeared fi'om the race, while in the northern 

 bijd the top of thehead is bald (Fig. 5, p. 153). Ordinarily the over- 



