No.604] SHOBTER Jh'TK Li:.s- AM) />/,s7 7 /o \ < 



249 



p. 30), in urging his substitute for thr ...m-e 

 scheme, largely on grounds of convenicn ^ Ivi^t 



(1912) as to the general usability of fli- l i i- -- ^ -i a 



"system of nomenclature" without eytological impli.'al ions. He 

 makes especially plain the undesirability of interpreting "ab- 

 sence" as a physical absence in the germ-cell, or (1915. p. 419) 

 as "a hole in a chromosome." In one respect, however. :\Iorgan.'s 

 discussion seems less clear than it might be, and this is in the use 

 of factor, in these articles, in a sense (the second here) which is 

 not that of the presence-and-absence scheme, with only vaguely 

 implied explanation of the distinction. Tt certainly is permissible 

 to speak of "the absence of a factor from tlie germ-plasm," 

 if we mean the hind of "factor'' i»i plied b,i the presence-and- 

 absence terminology. 



We must make it as clear as possil)le that factor {!) sometimes 

 means a potentiality and (2) sometimes menus a body, and that a 

 factor is assumed to be paired either (3) with its absence or (4) 

 with another (identical or different) factor. The combination of 

 (1) and (3), then, gives the presence-and-absence scheme, while 

 the combination of either (1) or (2) with (4) gives the scheme 

 used by iMorgan (]913; see also Castle, 1913) and other students 

 of Drosophila. General objeetioTis to :^Iendelian analy^s have 

 been based largely on r'onfiision of (1^ and (2\ which often 



analysis in general, or the ix-evriin'-and ah^,Mir>. inctlmd ,,f Men- 

 delian analysis, reciuiiv^ the nmwvo^-.wy and Hinvarrantcd as- 

 sumption involved in the romhinat ion of and ^T'. Prob- 

 ably this coTifusion is also harirrly rc^pon.-^ihle for tiie jictvl^tence 

 of another often diseredii.Ml notion, the i(h'a that ' ' M^'udrlians" 

 suppose their factors to be imlividually the I)a^is of .somatic char- 

 acters, rather than simply neces.sary elements in an interacting 

 (•oni])Iex wliich produces the characters. 



The term factor has, in genetic use. two distinct meanings, 

 which are continually interchani^cd or comhincd and often eon- 

 fused. It is essential to clearness in L'i'Uctic disrii.^sion that these 

 two meanings .should be carefully distinguished. The.se mean- 

 ings may be indicated by the following formal definitions: 



1. A genetic (Mendelian) factor is a property or characteristic 

 of the germ-plasm, more or less conveniently delimited for the 

 purpose of analysis of segregating heredity. 



2. A genetic (Mendelian) factor, or gene, is an actual material 



