12 APPLICATION OF PRINCIPLES OF HEEEDITY TO BREEDING. 



white variety. We know that the real difference between these 

 varieties hes in the fact that one of them produces red coloring mat- 

 ter and the other does not. We may therefore assume that in the 

 white variety a certain cell organ fails to perform a function which 

 the corresponding organ in the other variety does perform. We 

 may call this ' function which is performed in the red variety the 

 '^determiner" for red. In the white variety this determiner is 

 absent, although the cell organ which performs this function in the 

 other variety may be present in the white variety. In this variety 

 it fails to perform the function necessary to the production of red 

 coloring matter. 



We should not get the idea that red coloring matter is due wholly 

 to a single function of a single body, for such is probably not the case. 

 It may be necessary for several cell bodies to cooperate in the pro- 

 duction of this substance. In the white-flowered variety all of these 

 bodies may function properly except one, the failure of the one 

 body to perform its appropriate function being responsible for the 

 nonproduction of the red coloring matter. But when we are deal- 

 ing with a cross between these two varieties it is the one point in 

 which they difl^er that concerns us, and we shall use the word deter- 

 miner" to apply to this point of difference. Hence, we say that in 

 the one variety the determiner for red is present and in the other 

 it is absent. 



Although the determiner of a character is assumed to be a function 

 of a definite body, or of several such, we shall not attempt in what 

 follows to distinguish in all cases between these bodies and their 

 functions. In general, we shall represent the determiner for a 

 character by a capital letter, usually the initial letter of the name of 

 the character. Thus, capital R may be taken as the symbol of 

 the determiner for red coloring matter, but this symbol will be used 

 indifferently for the function which produces red and for the body 

 or group of bodies which has this function. For the absence of this 

 determiner in the white variety we shall use the corresponding 

 small letter. Thus, r may be considered in what follows as repre- 

 senting the absence of the function R, or it may be considered to rep- 

 resent the body present in the white variety that fails to perform the 

 function which is performed by the corresponding body in the red 

 variety. 



We are now ready to explain wh}^ the hybrid between a red and 

 a white variety of primrose produces two kinds of reproductive cells, 

 one like those of the red variety and one like those of the white — at 

 least, to offer an hypothesis that agrees with the facts. 



The red variety has inherited the determiner for red from two 

 parents. The condition of this determiner in the red variety may 



165 



