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THE AMERICAN NATURALIST [Vol. LIII 



In most instances it would appear as if the loss of all 

 the many factors concerned in the production of a single 

 plume takes place simultaneously, as is the case with most 

 meristic structures ; for usually the absences are complete 

 plumes. In some birds, however, two or three incom- 

 pletely formed or vestigial feathers occur between the 

 normal feathers of a row and the absent sockets, as if the 

 loss of the individual plume were taking place piecemeal. 

 This condition can be easily understood if we assume that 

 the constituent factors concerned do not all drop out 

 together, but follow some sort of succession. The factors 

 left at any time would then give rise to the part of the 

 feather for which alone they are responsible, and we 

 should get an imperfect or vestigial feather. In any 

 animal vestiges of a structure will continue to appear so 

 long as any of the factors concerned in the original struc- 

 ture remain. It is submitted that degeneration of any 

 complex structure never takes place in a gradual continu- 

 ous manner, as is usually supposed, but by successive 

 steps determined by the manner in which the factors drop 

 out; the appearance of continuity will however be con- 

 ferred if the steps are sm.all enough. 



If a sufficient number of individual ostriches were 

 gathered together it could easily be made to appear as if 

 degeneration in any of the recognized directions were 

 taking place in a slow continuous manner, for all stages 

 between the extremes could be obtained. Proceeding by 

 such a method however would give an erroneous impres- 

 sion of what is actually happening. For although all 

 stages do occur they are in reality disconnected, and each 

 stage has been reached in an individual quite irrespective 

 of the others, and represents a separate and distinct 

 germinal loss ; furthermore, in the same individual degen- 

 eration in any one direction proceeds quite independently 

 of the other directions in which the process is taking place. 

 It is not the wing as a whole which is undergoing degen- 

 eration, but the constituent parts of which it is made up, 

 each presumably represented by its own factors and be- 



