88 Outline of Genetics 



among individual differences, means the piling up of cumulative 

 factors in a given direction. Stop the selection and the old plants 

 with the small numbers of factors are allowed to survive, reproduce, 

 cross with the others, and eventually bring back the species to the 

 original average condition. 



One very seldom has any occasion to work out problems on 

 cumulative factors, since here the phenotypes do not show up as 

 clearly as they do in connection with the other factor types. Any 

 such problems, however, could readily be solved by some such 

 method as the following. Remember that we are dealing with a 

 dominance absent situation; and represent the number of doses 

 as exponents attached to the numbers which indicate the frequen- 

 cies of the different classes. 



AaBhCcXAABbcc. 



The A set gives a ratio of i with two doses : i with one dose, and 

 should be represented as i^: i'. 



The B set gives i with two doses: 2 with one dose: i with no 

 dose, and should be represented as 12:2^:1°. (i^*: iOX(i^:2': 1°) 

 equals 14:33 32:1^ 



The C set gives i with one dose:i with no doses, or 1^:1". 

 (14:33:32: 1') x(i':i°) equals 15:44:63:42; I^ The final result is i 

 with five doses: 4 with four: 6 with three: 4 with two:i with one. 



During the last decade, the mechanism of cumulative factors 

 has been invoked to explain a great many of the phenomena of 

 genetics. One noted instance of this will be worth considering, as 

 it has a very important bearing upon one of the fundamental con- 

 cepts in connection with the mechanism of inheritance. 



A few years ago geneticists might have been grouped into two 

 schools: "mutationists," who beUeved in the introduction of new 

 hereditary units by mutation alone, maintaining that the hereditary 

 genes were invariable and could not be modified by selection; and 

 ''selectionists," who beheved that the genes could be modified by 

 selection. The most prominent figure among the selectionists 

 was Castle, and the main experimental evidence upon which he 

 based his view was as follows. 



Castle (i) isolated a race of rats which had a black and white 

 coat pattern known as "hooded" (the black pigmented area hav- 



