J. E. DUERDEN 



133 



lapping of the feathers covers the naked areas, with the exception of the 

 legs ; but on hot days, when all the feathers stand on end and the wings 

 are out-stretched, the limited extent of the feathered areas (pterylae) 

 becomes a conspicuous feature. On the other hand, the valuable wing 

 plumes (remiges), for which the ostrich is prized, are almost unique 

 among birds in their high number, as many as 44 having been counted, 

 though the average is about 36 ^ It will be shown that the marked 

 absence of plumage in so many directions is largely a result of degenera- 

 tive losses, for survivors of a more heavily plumaged ancestral state are 

 yet to be found. 



-—II 



Fig. 1. Under surface of wing with plumes clipped off showing absence of feathers, 

 except the single incomplete row of under-coverts. The marginal row consists of 

 41 remiges and the last plume of the first and the second row of upper-coverts. The 

 row of under-coverts has one feather wanting at the beginning of the row and eight at 

 the elbow end. (I) claw on first finger or. bastard-wing ; (II) claw on second finger ; 

 (III) outline of third finger partly showing through the skin. 



It is deemed to be evidence of degeneration if plumes or other struc- 

 tural parts are absent from positions where by comparison with other 

 birds they would be expected to occur, more particularly if confirmatory 

 proof is forthcoming from their presence as survivals in other ostriches. 

 An ordinal succession of losses is presumed where all intermediate stages 

 between the extremes can be procured, the retrogressive process having 

 proceeded at different rates in different individuals, the various stages 

 representing successive steps in the retrogressive evolution of the race. 

 An attempt is made to interpret the changes in terms of the factorial 

 hypothesis^, though on account of the slow breeding nature of the 



1 Beebe (9th Anv. Rep. N. Y. Zool. Soc. 1905) states that the Albatross has 40 to 50 

 remiges, Rhea 28, Emeu and Apteryx 17, and Archaeopteryx 16. 



While the paper was in preparation there appeared an article by Prof. T. H. Morgan, 

 " Evolution by Mutation " (The Scientific Monthly, Vol. vii. No. 1, 1918). In this the 

 author briefly summarizes what mutation has to offer in the way of a contribution to 

 evolution, a timely exposition of which free use has been made. 



Journ. of Gen. ix 9 



