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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