No. 542] SOME ASPECTS OF CYTOLOGY 



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unit-characters are in fact dependent upon separate ma- 

 terial bodies or substances, and whether the chromo- 

 somes can be regarded as such bodies or the carriers of 

 such substances. 



Without entering upon the evidence in detail I shall 

 take it for granted that both these questions may be an- 

 swered in the affirmative. Accepting this (if only for 

 the sake of argument) how can such a conclusion be 

 reconciled with the ' ' action of the whole, ' ' and how form- 

 ulated so as to escape the pangen-theory? The latter 

 theory has fallen into discredit for two reasons. One is 

 because of the quasi-metaphysical assumption that the 

 1 'physical bases" or "determiners" of unit-characters 

 are organized, self-propagating germs. Let us lay this 

 assumption altogether aside as incapable of verification, 

 and think of the "determiners" in a more vague way 

 only as specific chemical entities of some kind. The 

 second and more serious objection lies against the notion 

 that the "determiners" are to be regarded as "bearers" 

 of the corresponding characters. This is a fundamental 

 error, as may be made clear, I think, by a specific illus- 

 tration. It has been proved that many ' ' unit-characters ' ' 

 are not units, but require for their production the co- 

 operation of several factors, as is shown with especial 

 clearness in the heredity of color. In such cases, as is 

 now generally recognized, we should not speak of "unit- 

 characters" but of unit-/ 'actors. Different "unit-char- 

 acters" come into view as particular unit-factors are 

 added to or subtracted from a given combination in the 

 zygote. The factor for gray color in mice, to take a 

 familiar example, operates by inducing a reaction of the 

 germ that can only take place in the presence of several 

 antecedent color-factors lower in the scale ; and the latter, 

 in turn, are only operative in the presence of still another 

 factor that is necessary to the production of any color. 

 The first and most obvious suggestion given by such 



