THEORIES IN PRACTICE 379 



relatively complete isolation of the factors for 

 quality we are seeking as will serve our purpose. 

 Such isolation might occur in the second genera- 

 tion, but it cannot be counted on to occur until 

 we have tried again and again, in each succes- 

 sive generation, using material that is little 

 less complex because a certain number 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 

 heredity deals is in reality made up of a thou- 

 sand factors. I do not mean to imply that the 

 number is just that; it is merely that a thousand 

 is a convenient round number for purposes of 

 our computation. 



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 be a thousand subordinate factors for 

 form of flower; a thousand others for texture of 

 petal; a thousand others for odor; yet another 

 thousand for hardiness; and so on for each and 

 every patent characteristic of flower and twig 

 and stem and root of the plant. In the aggre- 

 gate, let us say, there are a thousand different 

 "unit characters," each made up of a thousand 



