VARIATION. 



49 



recently found a case in fieldrats. The instances in which cross- 

 ing of two strains, which both had a recessive character of the 

 same aspect produced individuals with a character dominant 

 over that of both original kinds, can be found everywhere in the 

 literature of Mendelism. They are very important in that they 

 show, how new dominant characters can arise. 



Miss Douglas in her work with stocks has found more than 

 one set of such complementary factors. In one case some plants 

 were glabrous, instead of hairy, because they missed one gene, 

 and other plants were glabrous because another gene was 

 lacking, indispensable for hair-formation. A cross between two 



glabrous plants of these 

 different plants produces 

 hairy off-spring. Vilmorin 

 found a similar instance in 

 the pea, where two species 

 without waxy gloss produ- 

 ced hybrids which had it. 

 It is just as inadmissable in 

 such cases to say, that we 

 are dealing with the two fac- 

 tors which together produce 

 the dominant character, as 

 it is to say that one factor 

 which we know to be indis- 

 pensable for a certain pro- 

 cess in itself "determines" 

 this process and the re- 

 sulting character. 



The one or two genes 

 which we can study, 

 because they are indispen- 

 sable for the final result, 

 may be likened to links in 



Fa 



h $ ABAB At «B <* 



-"^^^-r^^-H * 



Fig. 6. 

 The effect of complementary genes. In 

 this case absence of either A or B breaks 

 the chain, and makes it impossible for 

 lower links, genes C, D, E and F to pro- 

 duce their effect upon the end result. 



an iron chain. (Fig. 6.) Any link in the chain may fancy itself to 

 be the link which holds up a weight, in reality all the links are 



