LUTHER BURBANK 



lines, which isolated, in the case of the individual 

 we selected, the particular character which we had 

 under observation more and more completely. 



Whereas, in a simple case of Mendelian her- 

 edity, where one pair of factors is in mind, there 

 is complete isolation of the recessive factor in one 

 case in four; in this complex case there is isolation 

 of groups of factors, and in one case among thou- 

 sands there may occur such relatively complete 

 isolation of the factors for quality we are seeking 

 as will serve our purpose. Such isolation might 

 occur in the second generation, but it cannot be 

 counted on to occur until we have tried again and 

 again, in each successive generation, using material 

 that is a little less complex because a certain num- 

 ber of disturbing factors have been segregated and 

 removed. 



We may perhaps illustrate the meaning of all 

 this a little more clearly if we suggest that each 

 so-called unit character with which Mendelian her- 

 edity deals is in reality made up of a thousand 

 factors. I do not mean to imply that the number 

 is just that; it is merely that a thousand is a con- 

 venient round number for purposes of our com- 

 putation. 



There would be, then, a thousand factors for 

 color combined to make up what we commonly 

 speak of as the unit factor for color; there would 



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