ii4 Plant Genetics 



environment, it must be admitted that JENNINGS' 

 work proves that unit characters may be modified by 

 selection. 



The evidence just given as to the modification of 

 unit characters seems rather conflicting. It will be 

 realized, however, that there are at present two leading 

 schools of geneticists, one believing that unit characters 

 may be modified by selection, and the other believing 

 that unit characters are invariable. This latter school 

 we have been referring to as Mendelians, but this is 

 hardly an accurate designation, since both schools are 

 really Mendelians. They are usually distinguished as 

 follows: "mutationists," who believe in the introduction 

 of new hereditary units by mutation alone and believe 

 that unit characters cannot be modified by selection; 

 and " selectionists," who believe that unit characters 

 may be modified by selection. In the previous dis- 

 cussions the views of the mutationists have been em- 

 phasized for two reasons: (i) mutationists are in the 

 majority at present; (2) their hypothesis gives a com- 

 prehensive and systematic basis for describing the facts 

 of inheritance. JENNINGS has recently tried to reconcile 

 the two schools by showing that their claims amount 

 to the same thing. Such a reconciliation is hopeful but 

 it may prove to be impossible. 



1. CASTLE, W. E., Heredity in relation to evolution and animal 

 breeding. New York. 1911. 



2. - , The inconstancy of unit characters. Amer. Nat. 

 46:352-362. 1912. 



3. EAST, E. M., The Mendelian notation as descriptive of physio- 

 logical facts. Amer. Nat. 46:633-655. 1912. 



4. JENNINGS, H. S., Observed changes in hereditary characters in 

 relation to evolution. Jour. Wash. Acad. Sci. 7: 281-301. 1917. 



