Mutations and Evolution. 
139 
differences they show in hereditary behaviour owing to the 
different factors they carry, or by actually counting their chromo¬ 
somes. The same is true of Metapodius (Wilson, 1909), in which 
some individuals were found having as many as six supernumerary 
chromosomes owing to irregular distributions of the Y chromosome 
in the reduction division, yet they were apparently indistinguishable 
from others with the normal equipment of chromosomes. This is 
in contrast to the conditions in plants, where, as shown in 
Chapter III, the extra chromosome is constantly associated with 
new external characters. Whether this is a general difference 
between plants and animals is unknown, but it may result from 
the fact that the cell unit in plants is a more rigid thing and less 
subject to regulation (in the physiological sense) from the structural 
point of view. 
In Drosophila then, the detection of the presence of an extra 
chromosome is only by means of the factors transmitted, or by 
direct observation of the chromosomes. In (Enothera there is 
also a difference in the hereditary behaviour, nearly all the 15- 
chromosome forms splitting at least into the two parental types. 
But in addition the external characters are directly affected. 
Although the breeding results with plants are necessarily much 
slower, yet it is hoped that the nature of the chromosomal 
differences between the various 15-chromosome types will 
ultimately be determined. 
It has-been necessary to dwell upon these results in Drosophila 
at some length, for although an animal it is obvious that the 
results concerning linkage, crossing over, lethal factors and the 
conceptions of the linear arrangement of factors in the chromosomes 
will fundamentally affect our conceptions regarding the basis of 
heredity and the constitution of the germplasm in plants as well. 
Indeed, this has already been the case. Once it is generally 
recognized that in the morphology and constitution of the chromatin 
lies the chief basis for the inheritance of differences, then we shall 
be on safe ground for future experimental progress. 
