CHAPTER VI 



THE PURE LINE 



1. THE UNIT CHARACTER METHOD OF ATTACK 



IN reducing any body of facts to a science, it is 

 first necessary to determine the underlying units 

 out of which the facts are made up. 



Chemistry was alchemy until the chemical ele- 

 ments were identified and isolated. Histology was 

 terra obscura until the cell theory brought forward 

 "cells" as the units of tissues. In the same way 

 there could be no science of genetics until the con- 

 ception was developed that the individual is a bundle 

 of unit characters rather than a unit in itself. So it 

 has come about that we now speak of inheritance as 

 applied to unit characters rather than to individuals 

 as a whole. 



Incidentally the fact that an organism is a com- 

 bination of many units makes it easy to account for 

 the wide diversity of forms found in nature, since the 

 addition of a single unit greatly increases the pos- 

 sible combinations in successive generations. 



Thus if three unit characters, A 9 B, and C, are 



present in each parent, for example, there would be 



six possible double combinations in the offspring, 



namely, A A, AB, AC, BB, BC, and CC. If now a 



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