82 Outline of Genetics 



strengthen greatly the conception of cumulative factors. 

 A summary of their conclusions is stated as follows: 



^'When one is dealing with quantitative characters, 

 that is, those produced by cumulative factors, he is 

 confronted by exactly the same principles of Mendelian 

 inheritance as have long been known to apply to quali- 

 tative characters. With quantitative characters, how- 

 ever, the problem is more complex, due chiefly to two 

 things: (i) we are usually deahng with more factors, 

 and factors cumulative in their operation; (2) the signifi- 

 cance of the breeding results is usually somewhat 

 obscured by the natural fluctuations due to response to 

 uncontrollable factors in the environment." 



In connection with the cumulative factor concept, a 

 modifying statement should be made as to the mecha- 

 nism involved. Heretofore it has been assumed that 

 we are dealing with numerous, separately inherited 

 factors, absolutely identical in their nature, cumulative 

 in their effect. No doubt one might regard with sus- 

 picion such a seemingly artificial mechanism. Probably 

 it would be easier to believe if it were modified in the 

 following manner. Instead of assuming that the numer- 

 ous factors are identical in function, we may assume 

 that each of these factors has its own peculiar function, 

 but that that function plays a part, directly or indirectly, 

 in developing the quantitative character in question. 

 For example, suppose height is the character. One of 

 the factors determines the development of numerous 

 nodes; another increases the amount of chloroj)hyll ; 

 another determines the size and \'igor of the root system; 

 another brings early germination and a long growing 

 season. Such factors, although not identical, will be 



