MENDELISM AND MUTATION 353 



ing facts in order to utilize them for the purposes of prediction, just as the chemi- 

 cal atom is a conception invented for the purpose of simplifying and making 

 useful observed chemical phenomena. As used mathematically, both the geneti- 

 cal factor and the chemical atom are concepts, but biological data lead us to 

 believe that the term factor represents a biological reality of whose nature we 

 are ignorant, just as a molecular formula represents a physical reality of a nature 

 yet but partly known. 



" With this distinction in mind, one may treat the factor or the atom from 

 two points of view, either as a mathematical concept or a physical reality. As 

 a mathematical concept it is the unit of heredity, and a unit in any notation must 

 be stable. If one describes a hypothetical unit by which to describe phenomena 

 and this unit varies, there is really no basis for description. He is forced to 

 hypothecate a second fixed unit to aid in describing the first. 



" The point at issue in this connection may be explained as follows : Characters 

 do vary from generation to generation, and the question to be decided is, how 

 much of this variation is due to the recombination of factors (considered now as 

 physical entities) and how much is due to change in the constitution of the 

 factors themselves . . . 



"... We believe there should be no hesitation in identifying the hypotheti- 

 cal factor unit with the physical unit factor of the germ cells. Occasional changes 

 in the constitution of these factors, changes which may have great or small 

 effects on the characters of the organism, do occur; but their frequency is not 

 such as to make necessary any change in our theory of the factor as a permanent 

 entity. In this conception biology is on a par with chemistry, for the practical 

 usefulness of the conception of stability in the atom is not affected by the knowl- 

 edge that the atoms of at least one element, radium, are breaking down rapidly 

 enough to make measurement of the process possible." 



Bibliography at end of Chapter XVIII. 



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