No. 584] HEREDITY AND ITS MEANING 



483 



varieties with which the fasciated type was crossed dif- 

 fered from it by several determinants, each of which was 

 transmitted independently though they every one affected 

 the development of fasciation. This illustration is not 

 one of a rare phenomenon. It is what geneticists find 

 constantly in their experiments. Presence or absence of 

 a particular character may depend upon the presence or 

 absence of a particular essential determinant, but, given 

 this determinant, sooner or later the investigator finds 

 several other determinants which modify the expression 

 of the character. The existence of these modifiers has 

 been the cause of a great deal of confusion in the analysis 

 of breeding results, but in reality the inheritance is sim- 

 ple. The experience that all investigators who have 

 worked intensively have had with them shows that prac- 

 tically all somatic characters are due to multiple determi- 

 nants in the germ cells. It merely depends on the rela- 

 tive difference between the germ plasms brought together 

 in crosses, how complex the resulting F 2 populations ap- 

 pear. Since even apparently simple characters are thus 

 due to complex germinal interactions, that results of 

 crosses made for the purpose of improving such intangi- 

 ble things as yield, size, quality, etc., should be complex, 

 is not astonishing. In the comparatively extensive expe- 

 rience that the writer has had in breeding tobacco, maize, 

 peas and beans the wide variability of the F 2 population 

 in crosses between distinct varieties leads him to think 

 that it is extremely common for such varieties to differ 

 qualitatively in even/ chromosome. Furthermore, the 

 relative complexity of the segregating populations is 

 much greater in tobacco than in corn and greater in corn 

 than in peas or beans. What can this mean but that when 

 varieties are found that differ qualitatively in all of their 

 chromosomes, the complexity of the result varies directly 

 with the number of chromosomes present. 



In Mendelian inheritance the number of actual types 

 (both homozygous and heterozygous) present in the F 2 

 population when all are represented is 3", and the number 



