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THE AMERICAN NATURALIST [Vol. XLV 



sis still known of any "character" in question. The 

 "units" or "unit-factors" stated in Mendelian work are 

 consequently quite provisory, depending essentially upon 

 the number of genodifferences in the special crossing. 

 Probably it may be discovered that several such "unit- 

 factors" for one character may also be elements for the 

 realization of quite other characters. If this be the truth, 

 then the present state of Mendelism, characterized by 

 the rapidly augmenting number of new "unit-factors" 

 demonstrated in the organization of different biotypes 

 able to hybridize, may be replaced by a period in which 

 many such unit-factors will be identified. At any rate 

 there is no reason to believe that the further Mendelian 

 analysis will augment the number of genes into absurd- 

 ity. The enormously increasing possibilities of combina- 

 tions by augmentation of the number of segregable genes 

 are a source of interest also in this connection. 



As to cases of hybridization, in which segregation and 

 combination do not suit the Mendelian "laws," it must 

 at first be stated that some apparent exceptions are prob- 

 ably caused by non-homogeneity of the initial material 

 for experiments. The experiments of Correns, Castle, 

 Miss Saunders, Tschermak and others have shown to 

 excess that phenotypes may seem totally "pure" and 

 nevertheless be heterogeneous (e. g., white flowering 

 stocks or albino mice). Thus constancy as to the pheno- 

 type of the progeny is no sure proof for genotypical 

 purity or unity. In discussing alternative inheritance 

 we meet with difficulties of the same nature as in regard- 

 ing fluctuating variability: the inadequacy of pheno- 

 type-description as the starting-point for genetic in- 

 quiries. 



Secondly, the more or less high vitality of the different 

 combinations of genes in F 2 may perturb the Mendelian 

 results, as Baur has illustrated; in other cases the dif- 

 ferent degree of facility with which the union of special 

 gametes is realized may influence the relative numbers 

 of representatives in the F 2 -generation, as Correns has 

 demonstrated. 



