J. E. DUERDEN 177 



of retrogression is the breaking-up followed by the diminution and 

 eventual loss of the fragments. Throughout we appear to have these 

 two processes, namely, slow successional reduction in size and somewhat 

 rapid fragmentation. 



Though representing a somatic absence, the factor for the break is 

 dominant over the factor for the presence of the scales, in the same 

 manner as the factor for the bald head patch in the northern ostrich is 

 dominant over that for the presence of the feathers in the southern bird. 

 Discussing the " presence and absence hypothesis " Morgan^ shows that 

 in certain cases a " present " factor may be recessive. " In the cross of 

 homed by hornless sheep, the horned condition dominates in one sex 

 and the hornless in the other. Here no matter which is considered as 

 a presence it must be conceded that in one sex or the other it is reces- 

 sive. The view that dominance of a factor proves its presence and 

 recessiveness its absence should therefore be abandoned." 



Degeneration and Adaptation. 



A brief enquiry may now be made into the adaptational significance 

 of the various degenerative changes which have been effected already in 

 the ostrich and of those which are still in progress. First, as regards 

 the losses of plumage. It is very questionable if those already carried 

 out, such as the loss of down, leg feathers and many of the wing coverts 

 and remiges, have any appreciable bearing on the welfare of the bird. 

 In life under arid conditions any reduction in the covering will assist in 

 radiation from the body, and may possibly be of some advantage in the 

 intense heat of the day. During the hottest weather chicks and adults 

 are often seen standing with their wings outspread and all the wing and 

 body feathers erect. On the other hand, loss of plumage will reduce the 

 protection against cold nights, though the overlapping of the present 

 feathers still suffices to clothe the bird when crouching. . 



The ostrich is highly responsive to humid cold as contrasted with 

 dry, cold weather. Its physiological state becomes greatly reduced during 

 prolonged, cold, rainy conditions, and chicks especially often succumb to 

 exposure, and even adults ; or, if the exposure be during the breeding 

 season, the cock may cease to pair and the hen to lay eggs, or both may 

 stop nesting. But this is scarcely a consequence of the scantiness of 

 the plumage, as might at first sight appear, but rather of the fact that 



1 Morgan, T. H., et al., The Mechanism of Mendelian Heredity, Holt and Co., New 

 York, 1915, p. 221. 



