UNIT CHARACTERS , 



Reality of Their Existence is Fundamental to Study of Evolution, But Has Never 

 Been Proved — Independent Variability of Parts and Independent 

 Transmissibility of Variations Open to Question' 



S. J. Holmes 



Associate Professor of Zoology, University of California, Berkeley, Cal. 



THE doctrine of unit characters 

 is one that has figured largely 

 in speculations on heredity and 

 evolution from the time of 

 Darwin to the present. According to 

 this doctrine an organism is a sort of 

 mosaic of parts each of which is depend- 

 ent for its development upon some kind 

 of discrete entity in the germ cell. 

 The germ cell is therefore considered a 

 complex of organic tmits more or less 

 independent of one another in their 

 activities and transmission. 



The unit character hypothesis is 

 founded on (1) the assumed independent 

 variability of the parts of an organism, 

 and (2) the assumption that characters 

 are capable of independent transmission. 

 Independent variability was appealed 

 to by Darwin in support of his hypo- 

 thetical gemmules, by De Vries in his 

 Intracellular Pangenesis, and especially 

 by Weismann who has adduced a 

 formidable array of facts in support 

 of this doctrine upon which he founds 

 much of his argument for the complex 

 organization of the germ plasm. 



"There a,re human families," says 

 Weismann, "in which individuals occur 

 repeatedly, and through several genera- 

 tions, who have a white lock of hair, in 

 a particular spot, on an otherwise dark- 

 haired head. This cannot be referred 

 to external influences, it must depend on 

 a difference in the getm, on one, too, 

 which does not affect the whole body, 

 not even all the hairs of the body, but 



only those of a particular spot on the 

 surface of the head. It is a matter of 

 indifference whether the white coloring 

 of the hair-tuft is produced by an 

 abnormal constitution of the matrix of 

 the hair, or by other histological ele- 

 ments of the skin, as of the blood- 

 vessels or nerves. It can only depend 

 ultimately on a divergently constituted 

 part of the germplasm, which can only 

 affect this one spot on the head, and 

 alter it, if it is itself different from what 

 is usual. On this account I call it the 

 determinant of the relevant skin-spot 

 and hair-group." 



"There must be as many of these 

 (determinants) as there are regions in 

 the fully-formed organism capable of 

 independent and transmissible varia- 

 tion, including all the stages of develop- 

 ment." 



Weismann has no quarrel with epige- 

 nesis^ as a theoretic possibility. The 

 complexity of the germ plasm is to be 

 measured by the amount of independent 

 variability occuring in the parts of the 

 organism. How great this amount is, 

 how many parts are capable of under- 

 going heritable changes independently 

 of the others is a question to be answered 

 only through extensive observation, 

 but one nevertheless capable, at least 

 theoretically, of being answered. 



weismann's argument 



Weismann argues with great plausi- 

 bility that the number of independently 



1 Read before a joint meeting of the zoological section, American Association for the Advance- 

 ment of vScience, and the American Genetic Association, at Stanford University, August 4, 1915. 



2 Older naturalists imagined that a minute but complete embryo must be preformed and in- 

 cased in either the egg or the sperm. In 1759 C. F. Wolff enunciated the doctrine of epigenesis 

 which, modified by later discoveries, is still accepted by the world of science. As at present 

 understood, it declares that there is no pre-existence of an organism as such, but that the embryo 

 is a new thing created as the result of the union of egg and sperm cells. — The Editor. 



473 



