246 



THE AMERICAN NATURALIST 



[Vol. LI 



p. 269) has remarked, are "attributes," no more to be separated 

 from the organism than are the properties of a chemical com- 

 pound from that compound. The factors of the presence-and- 

 absenee scheme, similarly, are inferred properties or attributes of 

 the germ-plasm, by whose behavior we explain the alternative 

 transmission of certain properties or attributes of the soma. 



Obviously an organism is not composed of "characters" — and 

 neither is its germ-plasm composed of "factors," so long as the 

 factors are those of the presence-and-absence scheme. Such 

 factors are nothing but characters of the germ-plasm, and, like 

 the characters of the soma, they are more or less conventionalized 

 in description. We have no warrant for projecting these con- 

 ventionalized descriptions back into the actual germ-plasm, and 

 assuming the presence and absence there of strictly correspond- 

 ing material units of segregation. 



The presence-and-absence scheme, when not encumbered with 

 non-essential hypotheses, is a strictly neutral instrument of genetic 

 analysis. If there is segregation in the formation of the germ- 

 cells, it is merely a matter of definition to state that a factor is 

 allelomorphic to its absence. That is, the assumption of segrega- 

 tion is the only assumption required by this scheme, which is the 

 logically simplest form of the "conceptual notation" (East, 

 1912) of genetics. 



Very special emphasis must be placed on the fact that the 

 "absence" is absence of a potentiality, without reference to the 

 presence or localization in the germ-plasm of any other poten- 

 tiality that may actually take its place. The allelomorphism of 

 the presence-and-absence notation is a logical opposition; when 

 it makes "A" and "no-A" allelomorphs, this involves no as- 

 sumption as to what may be physically opposed, in the chromo- 

 somes, to the physical basis of "A." 



If an "absence" a of a given factor A is always or commonly, 

 accompanied by an actual presence of a corresponding factor A\ 

 and we wish to represent this fact, it Js provided for by the 

 terminology of linkage; we may use Aa' and aA' , since the 

 presence-and-absence scheme makes no assumptions as to the 

 structure of the germ-plasm. It is obviously simpler to write 

 simply A and A', or A and a, for the two factors, and conveni- 

 ence may justify this practise ; we should note, however, that in 

 thus abandoning the presence-and-absence terminology we intro- 

 duce a second assumption, that of actual factor-to-factor opposi- 

 tion or allelomorphism. This assumption is, of course, in view of 

 all the evidence, a highly probable one, and especially convenient 



