152 Degeneration in the Ostrich 



Were an adaptive significance for the loss of the leg coverts to be 

 sought for it might possibly be found in connection with the increased 

 size of the wing plumes during the middle chick stage. While the 

 wing plumes are small the outer surface of the leg is exposed, and 

 would be uncovered were it not for its own contours. As the chicks 

 become older the wing-quills grow longer and, in extending backwards 

 and downwards, serve to cover the fore-leg, when it may be held there 

 is no further need for the protection from the contours. 



Admitting an adaptive value of the above nature does not however 

 imply any direct connection between the adaptation and the loss of the 

 contours. Without doubt the relationship suggested is incidental, while 

 the dropping out of the feathers is part and parcel of the general scheme 

 of plumage retrogression going on in the ostrich. But the method by 

 which the loss is achieved is of an altogether different nature from that 

 followed in the case of the coverts and remiges. The germinal factors 

 concerned with the production of leg contours would appear to retain 

 their normal activity until the chicks- are about six months old, and 

 then by for the greater number lose their effectiveness and the feathers 

 fall out, all within a brief period, due to a common influence acting 

 simultaneously, much in the same manner as the human hair sometimes 

 falls off after a severe attack of fever or of influenza. Two explanations 

 seem possible. Either the feather-producing factors directly lose their 

 effectiveness with the late chick stage, or they are inhibited by some 

 physiological influence arising in the chick. Whichever be the case the 

 original factorial change leading to the loss was presumably effected in 

 the germ plasm, and is of the nature of a retrogressive mutational 

 variation, although expressing itself late in ontogeny. 



An ontogenetic loss of this nature has manifestly a wholly difierent 

 significance from the successive disappearance of the individual feathers 

 belonging to the rows of coverts and row of remiges, where each in turn 

 retrogresses in a slow, successional fashion. The crural feathers drop 

 out nearly simultaneously from a whole area, an entire pteryla is in- 

 volved, and the losses are complete feathers which have not undergone 

 any preparatory degenerative change, either as a whole or in their 

 constituent parts. The one is as typical an illustration of continued, 

 retrogressive change, with a definite trend, as could well be conceived, 

 while the other is as direct an example of discontinuous, retrogressive 

 change, unconnected with any other; yet both are to be found in 

 the same organism, and represent the same tendency towards plumage 

 reduction. 



