J. E. DUERDEN 139 



beginning of degeneration, it is the feather as a whole which becomes 

 reduced in size, all its structural parts becoming smaller in due propor- 

 tion. Manifestly in this case the common controlling influence is under- 

 going some slow degradation, and all the constituent factors are corre- 

 lated with it ; unless, with Morgan, we think of the retrogressive changes 

 as being due to other genes, multiple modifying factors, rather than the 

 principal one. 



In the later stages of degeneration where, in addition to continued 

 diminution, actual loss of the structural parts of the feather occurs, it is 

 clear that the separate factors of the feather are losing their potency, 

 and in the end cease to gain somatic expression. The retrogression for 

 these proceeds according to the following sequence : first, a weakening 

 and loss of the factors concerned with the shaft ; second, of those repre- 

 senting the barbules; and third, of those responsible for the barbs. 

 Thus two influences seem to be at work : one resulting in a weakening 

 of the factor or factors which control the feather as a whole, and the 

 other, and later one, in a weakening and ultimate loss of those which 

 determine the structural details. Where retrogression is in progress it 

 by no means follows however that a factor is lost to the germ plasm as 

 soon as it fails in somatic expression. Evidence from what seem to be 

 " reversions " supports the idea that the factors may continue in the 

 germ plasm after losing their ordinary power of somatic manifestation, 

 and presumably they continue their course of degradation until, as it 

 were, they finally atrophy. 



Whatever the degenerative influences within or acting upon the germ 

 plasm may be, they proceed slowly and continuously for the race as a 

 whole, but with some variability in the rate of action in different strains. 

 All intermediate conditions between the extreme stages are easily forth- 

 coming ; there is no suggestion of any abruptness or break between one 

 and the other. If, in the case of the diminution in size of the individual 

 feather, we hold that the changes are the expression of a single con- 

 trolling factor the latter must be undergoing a gradual reduction in 

 power, whereas if the loss of a multitude of constituent factors is con- 

 cerned the expression of their individual loss is so small as still to leave 

 the impression of continuity ; even if modifying inhibitory factors are 

 invoked their action must be effected in the same slow, sequential 

 manner. The factorial view adopted has obviously no pragmatical 

 bearing upon the results of degeneration, as contributory to the evolu- 

 tion of the ostrich ; but the hard fact of continued, rectigrade mutations 

 has to be faced by Mendelism. 



