J. E. DUERDEN 167 



latter naturally brings about the former, but the process has taken place 

 more rapidly in some areas than in others, and so produced a regional 

 diflfeience. They correspond with the degenerative results among the 

 coverts, where the constituent parts disappear in the same sequence 

 and the area of the fore-wing provided with feathers gradually becomes 

 less and less. On the view that the plumage of the ostrich as a whole 

 is retrogi'essive, proceeding for the most part according to some definite 

 order, the down might be expected to be the first to suffer, and the 

 weak representation at the present time serves to confirm this. The 

 many intermediate conditions still surviving indicate that the process 

 has been a continuous one, while the extremely sparse remnants in 

 some birds suggest that in time the covering will be entirely lost 

 in certain strains and ultimately for the entire race. Embryos of all 

 ages fail to afiford any fuller ontogenetic traces of the down, so that 

 when absent in the chick and adult it may be presumed that the factors 

 concerned have altogether disappeared. 



Digits of the Wing. 



The wing of the ostrich is usually described as degenerate compared 

 with that of other birds. But the term can be applied only in a super- 

 ficial sense, as referring to its size in contrast with that of the legs and 

 body. The disproportion is such that in their present state the wings 

 could never have been effective for flight, even if the loose plumes had 

 enabled them to offer any resistance to the air. As a matter of fact, 

 the detailed structure of the wing, particularly as regards the digits 

 and their claws, proves it to be less degenerate than that of perhaps 

 any other living bird, and to be more nearly related to the supposed 

 reptilian ancestor of the class Aves. Evidence has already been pre- 

 sented to show that the fore-wing is probably still undergoing a gradual 

 reduction in size, in correlation with the loss of plumage. It seems 

 likely, however, that any change which occurs takes place in the wing 

 as a whole, and is not necessarily accompanied by loss of its separate 

 structural parts, as appears to be largely the case with Apteryx. In 

 plumes also it is shown that a reduction in size as a whole takes place, and 

 only towards the end a loss of the constituents. The two processes need 

 to be kept distinct in retrogressive studies, in the same manner that 

 we distinguish between progressive increase in size and in complexity. 

 As regards the adult ostrich the third digit and its plumes are the only 

 structures which seem to be in a degenerative phase at the present 



