352 INTRODUCTION TO CYTOLOGY 



believed to be due to a factor mutation in the very young bud, the entire 

 product of the bud— branch, flower, or flower cluster — has a new geno- 

 typic constitution and exhibits an appearance often strikingly different 

 from that of the other branches or flowers of the plant. Such a factor 

 mutation occurring in a partially developed shoot or organ results in a 

 chimera, in which a distinct portion of the mature structure, commonly a 

 sharply defined sector, differs genotypically and in appearance from the 

 other portions. By some geneticists vegetative mutations are thought to 

 be due to a somatic segregation of allelomorphic factors in a heterozygous 

 individual and their consequent independent activity in different por- 

 tions of the body. 



The nature of the change which may thus occur in the gene is unknown, 

 since the nature of the gene itself is entirely a matter of conjecture. 

 Although it has been suggested that the gene, because of its relative 

 stability, may be simply a molecule, it is more probably a more complex 

 colloidal aggregate, possibly enzymatic in nature, which is capable of 

 growth and division. As such it could not be expected to be absolutely 

 stable, as some geneticists have thought, but changes would in all likeli- 

 hood take place occasionally by addition, loss, or rearrangement of the 

 constituent atoms of the molecule. The probable rate of change of the 

 genes in Drosophila has been calculated by Muller and Altenburg (1919). 

 What the agencies are which cause such changes is also unknown. Some 

 evidence has been brought forward to show that genes may be modified 

 by external influences, but by many it is regarded as of very doubtful 

 value. The stimuli to which the genes respond by undergoing some 

 constitutional change are probably for the most part internal ones. 

 Although the number of ways in which a gene may change is limited by 

 its own organization, the possible changes are nevertheless numerous, 

 so that it is very probable that many variations which constitute initial 

 steps in evolution originate in this manner. 1 



Conclusion. — The following paragraphs are quoted from East and 

 Jones (1919, pp. 76 ff.): 



'The relation between fact and theory in the Mendelian conception of 

 inheritance is this: Various kinds of animals and of plants were crossed and the 

 results recorded. With the repetition of experiments under comparatively 

 constant environments these results recurred with sufficient regularity to justify 

 the use of a notation in which theoretical factors or genes located in the germ cells 

 replaced the actual somatic characters found by experiment. Later, the ob- 

 served behavior of the chromosomes justified localizing these factors as more or 

 less definite physical entities residing in them. Now the data from the breeding 

 pen or the pedigree culture plot and the observations on the behavior of the 

 chromosomes during gametogenesis and fertilization are facts. The factors are 

 part of conceptual notation invented for simplifying the description of the breed- 



1 See the discussion of these points by Conklin (1919-1920). 



