42 Mutations and Evolution. 



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 Drosophilatheu, 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. 



