68 Outline of Genetics 



fittest. It follows from this that there has been a struggle for 

 existence among unit characters. Those dominant unit char- 

 acters which are "fit," which serve better to adapt the organism 

 to environmental conditions, are the ones to survive; while 

 "unfit" dominant unit characters are eliminated with the elimi- 

 nation of the organisms that contain them. (The same reasoning 

 would not apply to the recessives, which are "protected" from 

 natural selection when they occur in heterozygotes.) These 

 dominant unit characters which exist today are, for the most part, 

 "survival" characters, being important in the economy of the 

 organism since they are serving to adapt it to the environment. 

 In addition, there must of course be a number of "indifferent" 

 characters, which cannot be construed as adaptations; these 

 have persisted simply because, since they have neither positive 

 nor negative survival value, there has been no reason to eliminate 

 them. Since most of the dominant unit characters have positive 

 survival value, it follows that anything which prevents the expres- 

 sion of these dominant unit characters must have negative sur- 

 vival value. This is exactly what the dominant inhibitory factor 

 amounts to; it is something which prevents the expression of a 

 dominant unit character. One is forced to the following conclu- 

 sion. Although dominant inhibitory factors may have come into 

 existence just as frequently and numerously as the other types of 

 factors and determiners, most of these inhibitors would have been 

 ehminated through natural selection on account of their negative 

 survival value. The few dominant inhibitory factors which per- 

 sisted would be those which inhibited "indifferent" characters. 

 Color in grains of corn is doubtless an "indifferent" character. 



(There are two types of things which are fairly common and 

 which might be confused with inhibitory factors: (i) simple 

 determiners which produce such characters as to "mask" other 

 characters without really inhibiting other determiners; (2) "lethel 

 factors" which will be explained on p. 69.) 



On the other hand, it is not surprising to find that supple- 

 mentary factors are rather common. The supplementary factor 

 carries further a reactioh which has been brought up to a certain 

 point by some other force. Exactly this sort of mechanism must 

 play a large part in the ontogeny of most organisms. The litera- 



