THE IDEA OF UNIT CHARACTERS 369 



dependently heritable unit characters is not inconsistent with, 

 but rather corroborates Weismann's picture of an inheritance as 

 composed of numerous sets of determinants or primary constitu- 

 ents, each corresponding to an independently variable and 

 heritable structure. It is quite possible that the germ-cells of 

 the hybrids of two distinctively contrasted parents do not 

 separate into two sets bearing " pure " dominant determinants 

 and " pure " recessive determinants, but that the practical 

 " purity " is wrought out by a process of germinal selection. 



However this may be, the facts of Mendelism lead us to a 

 renewed confidence in the relative independence of unit char- 

 acters. It looks as if a unit character sometimes behaves like 

 a radicle in chemistry ; it can be replaced en hloc by another, 

 but it cannot compromise with that other. " The outlook," 

 as Bateson says, " is not very different from that which opened 

 in chemistry when definiteness began to be perceived in the 

 laws of chemical combination." 



While the idea of unit characters, which is well backed up by 

 facts, tends to clearness, it must be cautiously worked with. 



1. There are only certain peculiarities of the organism of which 

 it can be said that they demonstrably behave as unit characters. 

 But this may simply mean that in regard to other peculiarities 

 we have as yet been unable to discover the appropriate 

 contrasted crossing which would bring out the characteristic 

 behaviour of allelomorphs. 



2. There are also cases— g.g. of inbred crosses of Cheviot 

 and Leicester sheep — where the conception of unit characters 

 remains unverified. It seems to us safer, at present, to say 

 that some but not all the peculiarities of contrasted types behave 

 as " unit characters." 



3. Correns points out that what seems at first sight like an 

 independent unit character may turn out to be nothing more 

 than the necessary consequence of another character. " Thus the 

 wrinkled appearance of the sugar-maize seed, in contrast to the 



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