CHAPTER VII 



THE FACTOR HYPOTHESIS 

 1. THE HEREDITARY UNIT 



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 elements 

 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 conception was de- 

 veloped that the individual is a bundle of unit char- 

 acters rather than a unit in itself. So it has come 

 about that geneticists speak of inheritance as applied 

 to unit characters rather than to individuals as a whole. 



The apparent somatic unit characters, like the 

 color of the seed-coat or the length of the vine in 

 Mendel's peas, are conditioned by other intangible but 

 nevertheless real germinal units or determiners which 

 give rise to them. Mendel was apparently unaware of 

 the existence, in certain cases at least, of compound de- 

 terminers. His experiments led him to believe that 

 each character depends upon only a single determiner 

 for the reason that he worked on characters severally 

 belonging to different parts of the plant, but it has 



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