J. E. DUERDEN 



149 



variations which Johannsen and Jennings encountered will occur. From 

 the variations between one chick and another not much that is decisive 

 can be gained where the parents are admittedly of mixed ancestry, but 

 when we find variations between one side and the other in so many 

 features it becomes clear that we are encountering some factorial 

 irregularity. 



The diminutive and vestigial feathers are somatic evidence of factorial 

 degi-adation, and it is submitted that the variations as between one wing 

 and the other of the same bird reveal that the factors are approaching 

 the limit of theii* power of expressibility, and that in this state they may 

 or may not act with strict regularity for different individuals or for the 

 two sides of bilateral structures. It seems trite to repeat that, however 

 pure the breeding may be, uniformity of characters can not be expected 

 if the factors themselves are in a changing or very weakened phase, but 

 only when they are fixed and stabled 



Table II represents the nimiber of plumes on a hatching of North 

 African chicks of which the parents have the average number of plumes. 

 Both the cock and the hen are imported northern birds of mixed ancestry 

 and the results show an approximation to the parental numbers, though 

 with slight variations above and below, but by no means approaching 

 the wide extremes characteristic of the race. The numbers are wholly 

 different from those of the former series, except that the highest, 39, in 

 the one is the lowest in the other. 



^ In his lecture, " The role of Selection in Evolution," before the Washington Academy 

 of Sciences in 1917, Prof. W. E. Castle remarks that the pure line principle "does not fit 

 in with the observed facts as regards the effects of selection in the majority of the domes- 

 ticated animals and cultivated plants, nor even with the behaviour of certain characters 

 n self-fertilised plants and asexually propagated animals." He refers to the genetic 

 variations encountered by Jennings with Dijitigia, his own with guinea-pigs, rabbits and 

 rats, and that of various workers with asexually reprodacing plants. 



Journ. of Gen. IX 10 



