76 INBREEDING AND OUTBREEDING 



forget which part of this diagrammatic representation 

 of the heredity mechanism is fact and which part is 

 theory, for confusion between the two has led to a regret- 

 table controversy over a point which is of paramount im- 

 portance in any discussion of inbreeding and outbreeding 

 the stability of inherited factors. 



The relation between fact and theory in the Mendelian 

 conception of inheritance is this: Various kinds of ani- 

 mals and of plants were crossed and the results recorded. 

 With the repetition of experiments under comparatively 

 constant environments these results recurred with suf- 

 ficient regularity to justify the use of a notation in which 

 theoretical factors or genes located in the germ cells re- 

 placed the actual somatic characters found by experiment. 

 Later, the observed behavior of the chromosomes justi- 

 fied localizing these factors as more or less definite physi- 

 cal entities residing in them. Now the data from the 

 breeding pen or the pedigree culture plot and the ob- 

 servations on the behavior of the chromosomes during 

 gametogenesis and fertilization are facts. The factors 

 are part of a conceptual notation invented for simplifying 

 the description of the breeding facts in order to utilize 

 them for purposes of prediction, just as the chemical 

 atom is a conception invented for the purpose of simpli- 

 fying and making useful observed chemical phenomena. 

 As used mathematically, both the genetical 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 molecu- 

 lar formula represents a physical reality of a nature yet 

 but partly known. 



With this distinction in mind, one may treat the fac- 



