322 



THE AMERICAN NATURALIST 



[Vol. LIII 



V 



"Hereditary determiners or factors fluctuate regularly 

 and frequently, if not indeed usually, and in high correla- 

 tion with somatic characters." 



In a certain measure this statement may be looked upon 

 as opposed to that with which section 1 opens, but no one 

 would maintain either the one or the other to be the ex- 

 clusive state of the germ plasm, hereditary determiners 

 or factors of all animals and plants. We have abundant 

 evidence that the germ plasm is remarkably conservative 

 for some forms of life (persistent types) while in others 

 it may fluctuate or change frequently (Drosophila) ; also, 

 it is not unreasonable to expect that at any one period the 

 factors for certain parts of an organism may remain fixed 

 while for others they may be in a state of change. We 

 have seen that the factors controlling the structural de- 

 tails of the ostrich plume are peculiarly constant, but the 

 endeavor will now be made to establish that those for the 

 wing feathers numerically, as well as for certain other 

 parts of the bird, are undergoing regular and frequent 

 changes and in determinable directions. 



By zoologists the wing of the ostrich is usually regarded 

 as degenerate, on account of its small size compared with 

 the body and legs and the practical absence of any cover- 

 ing of feathers on its inner or under surface. Certain 

 studies recently made have given good reason for con- 

 cluding that in many other less obvious respects it is still 

 undergoing degeneration. The full details upon which 



3 The three statements under section B are obviously considered by Dr. 

 Pearl to apply specially to Dr. Castle's claims in connection with his ex- 

 periments on piebald rats, a condensed account of which appears in the same 



