148 Degeneration in the Ostrich 



As will be seen later in connection with the other paired structures 

 undergoing degeneration, particularly the claw on the fourth toe and 

 the scales on the third, a similar variation often appears between the 

 two sides, and would also seem to find its explanation in the fact that 

 the genetic factors concerned are in a changeable condition. Where 

 factors are undergoing retrogression, whatever their nature, a stage will 

 naturally be reached so near the margin of somatic expression or non- 

 expression that uncertainty will occur as to whether they gain expression 

 or not, varying'conditions in the soma perhaps having some influence. 

 The result will be particularly manifest in the case of normally paired 

 structures, if they should appear on one side of the body and not on the 

 other. Where structural features are in a degenerative phase slight 

 variations in the progeny from the same parents, as well as between the 

 right and left sides of an individual, will therefore not necessarily be 

 evidence of germinal impurity, but an indication of factorial weakness ; 

 neither will they be testimony to environmental influences, such as 

 manifest themselves among the individuals of a pure genotypic line, for 

 normally any numerical fluctuations are germinal \ 



The conclusion reached has obviously an important bearing upon the 

 general question of germinal purity and its manifestation in the soma. 

 Only when the genetic factors are fixed and stable can we hope for 

 somatic purity, or be in a position to extract genotypic lines altogether 

 pure in their characteristics. Contrariwise, where genotypic purity is 

 attained it may be deemed to be evidence that the factors are in a 

 settled state, as in Johannsen's well-known experiments with beans 

 and those of Jennings with Paramoecia, where the characteristics for 

 each line were found to be constant and only environmental varia- 

 tions appeared. One advantage in connection with genetic studies on 

 the plumes of the ostrich is that the differences are numerical and there- 

 fore particularly concrete, and the numbers do not vary with external 

 influences, nor any changes introduced in the course of the life-time. 

 Moreover, by dealing with concrete numbers none of the fluctuating 



1 Dr A. F. SUull (Amer. Nat. Vol. li. 1917, p. 365) refers to the fact that Hyde has 

 recently reported a case in Drosophila in which the two X chromosomes appear to have 

 remained undivided, going to opposite daughter cells, and resulting in the production of 

 right and left eyes of different sex-linked colours. "While one can conceive of occasional 

 cases of the failure of chromosomes to divide during mitosis, thereby giving rise to 

 asymmetry, it can scarcely be held for the ostrich where bilateral irregularity is so 

 frequent. Factorial degradation to near the margin of expression appears a more likely 

 interpretation, especially when considered in conjunction with the general facts of degene- 

 ration presented by the ostrich. 



