188 Degeneration in the Oi^trich 



The only feasible explanation which can be advanced for the rare 

 presence of second and third row under-coverts is that the ancestors of 

 the ostrich were, like modern flying birds, provided with a covering of 

 feathers to the under or inner surface of the wing, though in nearly all 

 cases this is now naked except for the single row of under-coverts, itself 

 a disappearing quantity. 



We appear to be justified in assuming that the loss of plumes from 

 under the wing has been going on continuously in the ostrich race for a 

 long period, but at a different rate in different individuals. In by far 

 the majority of birds all have disappeared with the exception of the first 

 row of coverts. The germinal factors for a regular succession in the 

 second row still persist in one strain, and for a less number of the third 

 row, while in other strains the factors for odd members only remain. 

 No reasonable doubt can be entertained that the stages still surviving 

 represent in a general way the ordinal succession according to which the 

 factorial losses have proceeded and are still proceeding for the race as a 

 whole, the factors for single plumes from one end of a row first, and th^n 

 from both ends. The vestigial feathers frequently found next the 

 ordinary ones reveal that the losses are not effected as complete plumes, 

 but that each passes through a definite series of degenerative stages 

 before its final disappearance. The loss proceeds by a gradual reduction 

 in size, accompanied in the end by a breaking up and loss of the con- 

 stituent parts. 



An individual ostrich plume is made up of a number of constituent 

 parts — quill, shaft, barbs and barbules. The minute structural details 

 of all these vary greatly in different strains, the differences determining 

 the commercial value of the plume, and therefore calling for intensive 

 study on the part of the ostrich breeder. Selection in breeding, now 

 extensively carried on, has fully established the fact that the diversities 

 behave as independent characters, and must therefore have some separate 

 factorial representation in the germ plasm. As a consequence the fac- 

 torial constitution of even a single ostrich plume must be highly complex. 

 A moment's reflection however suffices to show that the complexity 

 generally acts in a unified manner. Thus a feather appears and develops 

 in its entirety, and also responds as a whole to changing nutritive con- 

 ditions, as if some common influence were controlling its component 

 factors. The same cause it must be which determines the relative sizes 

 of the plumes in a row where, as in, the case of the remiges, the middle 

 members are the largest, and a gradual reduction takes place towards 

 each end. In the case of the diminutive feathers which represent the 



