34 
Mutations and Evolution. 
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 
pome 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. 
(To be continued). 
