276 Symmetry [CH. 



colony just as bud-sports are on plants, and hence it is by 

 no means unlikely that the differences between the avicularia 

 may be due to allelomorphic segregation. 



Asymmetry and Variation. 



Stripped of all that is superfluous and of all that is 

 special to particular cases, genetics stand out as the study 

 of the process of cell-division. If we had any real know- 

 ledge of the actual nature of the processes by which a cell 

 divides, the rest would be largely application and extension. 

 It is in cell-division that almost all the phenomena of 

 heredity and variation are accomplished. 



Heredity being a special case of symmetrical division, 

 genetic variation is the consequence of asymmetrical division. 

 The cause of the asymmetry may lie far back in the history 

 of the tissue or of the germs. The germ-series may for 

 instance be represented as 



1. A 



2. A A 



/\ /\ 



3. A a A A 



where the division in which the variant, a, first appears is 

 actually an asymmetrical one ; or we may imagine the 

 process occurring in such a way that the first appearance of 

 a is produced by the division of an A cell into a and a 

 thus: 



1. A 



2. A A 



/\ /\ 



3. a a A A 



In the latter case the asymmetry was introduced into 

 the series at some division antecedent to that at which the 

 a form actually appeared. The question whether one of 

 these schemes is correct as a representation of the natural 

 processes, to the exclusion of the other, is a not unimportant 

 one, but there is no adequate ground for a positive answer 

 as yet. 



In comparing the somatic differentiations with genetic 



