114 ANNALS NEW YORK ACADEMY OF SCIENCES 



absence applies to another factor that concerns the change from chocolate 

 to black. A combination of the two hypotheses seems to me to be more 

 useful than the adoption of either alone. Moreover we can see, on the 

 view of the allelomorphic relation of the factors that I have used here, 

 why in some cases one and in other cases the other way of expressing the 

 facts should be used. 



UNIT CHARACTERS AND FACTORS IN MENDELIAN INHERITANCE 



I understand by the term unit character any particular structure or 

 function that may appear in heredity independent of other characters. 

 Such unit characters may in themselves be extremely complex and include 

 the possibility of further splitting up. By factor I understand some 

 special condition in the germ plasm whose presence is necessary for the 

 development of a particular unit character which in its absence fails to 

 develop. There is unquestionably a tendency in much of the Mendelian 

 writing to identify the unit character with one only of the factors whose 

 presence is necessary for its development. This view seems to be as un- 

 necessary in the present state of our knowledge as it is unproven by the 

 evidence at hand. It is this point of view that underlies the entire Weis- 

 mannian conception of the process of heredity. The acceptance of tha 

 theory of factors does not seem to me to lead necessarily to such a con- 

 clusion ; for, while a factor may be essential to the development of a par- 

 ticular part, it may be only one condition, several or many conditions 

 combining to produce the effect, the absence of any one of them leading to 

 the same result. Thus, while it may be that a particular chromosome or 

 a small part of a chromosome is essential for the formation of a unit 

 character, it may be that all the other chromosomes and all or many of 

 the elements that constitute the cell also take part in the final elaboration 

 of the special organ in question. This may represent the extreme case, 

 and there may be all degrees in which different parts of the cell combine 

 to produce any given organ. The essential point is that the acceptance 

 of the factor hypothesis in heredity by no means leads to the conception 

 of each character being located in a special unit or biophore or pangene in 

 the egg, for, as I have said, many parts may be equally essential for the 

 production of every part. 



Concerning the mechanism of segregation in the germ-cell we know 

 very little at present. Many facts seem to point to the conclusion that 

 the simplest solution of segregation is that a sorting-out of material par- 

 ticles takes place during those processes that come under the general head 

 of maturation in the egg or spermatozoon. This sorting process involves 



