THE METHOD OF EUGENICS 25 



that when both parents have low grades of a trait-complex 

 the children will have low grades of that complex. 



The matter of dependence of a character on a determiner 

 or its absence is of great importance and is not easy to anti- 

 cipate. For instance, long hair as in angora cats, sheep or 

 guinea pigs is apparently not due to a factor added to short 

 hair but rather to the absence of the determiner that stops 

 growth in short-haired animals. One can only conclude 

 whether a character is due to a determiner or to its absence 

 by noting the effect of breeding likes in respect to the given 

 trait. If all offspring are like the parents in respect to a trait, 

 the trait (if simple) is probably a negative one. But if the 

 offspring are very diverse, the trait (if simple) is probably 

 due to a positive determiner and the germ cells of the parents 

 are of two kinds; some with and some without the deter- 

 miner. 



The determination of unit characters is complicated by 

 the fact that a character due to a simplex determiner often 

 differs from one due to a duplex determiner. In the former 

 case the character is slow in developing and frequently 

 fails of reaching a stage of development found in the latter 

 case. The offspring of red and black-eyed birds may have 

 at first a Ught iris which gradually darkens. This fact is 

 spoken of as the imperfection of dominance in the simplex 

 condition. 



Despite the difficulties in analysis of units of heredity and 

 despite the compUcations in characters it is possible to see 

 clearly the method of inheritance of a great number of 

 human traits and to predict that many more will become 

 analyzed in the near future. 



