Foundations of the Mutation Concept. 9 



Just as the cell theory of organic structure has long since 

 passed into the realm of fact, so in a sense has the chromosome 

 theory of inheritance. We need mention only three of the most 

 striking cases where a definite relationship has been established 

 between a particular chromosome complex and a certain set of 

 external characters. These are (a) the sex chromosomes, now 

 known to show dimorphism in a wide range of animals, and recently 

 discovered in a liverwort (Allen 1919) ; (b) the aberrant chromosome 

 conditions in the (Enothera lata series of forms ; and (c) the non- 

 disjunction phenomena in Drosophila. In all these cases, 

 without obscuring the issues by introducing the concept of 

 " causality," it is definitely known that a certain set of external 

 features of the organism is associated with a certain structural 

 complex of its nuclei, and this complex has been passed on by 

 mitotic division from its inception in the fertilized egg. This is 

 not hypothesis but fact. (Enothera lata whenever it is examined 

 /some 50 individuals have had their chromosomes counted) invariably 

 shows the extra chromosome in its mitotic figures. The 

 spermatocytes or the embryonic tissue of the males in various 

 groups of insects show the characteristic male constellation of 

 chromosomes, i.e., with an X or an XY group in addition to the 

 autosomes, while the females show a corresponding XX group. 



While we must therefore regard the " chromosome theory," 

 like the cell theory, as an established fact up to a certain point, so, 

 again like the cell theory, must we also regard it as subject to 

 qualifications and limitations. 



Since we are not here discussing the general arguments for a 

 more minute and detailed relationship between the chromatin and 

 the complex of inherited characters, this aspect of the subject may 

 be left for the present, with the remark that in the three classes 

 of cases cited a definite relationship which may be classed as 

 hereditary has been clearly established between particular 

 external features and particular chromosome complexes of the 

 nuclei. 



Prom the standpoint of the mutation concept, (Enothera lata, 

 while a critical case in demonstrating the occurrence of germinal 

 changes, differs in several respects from the most frequent 

 and widespread type of mutation, i.e., the type which results in a new 

 Mendelian character. The latter type of change will be considered 

 in greater detail in connection with the origin of dominant and 

 recessive Mendelian characters! mass mutations and parallel 



