MENTAL CHARACTERS IN MAN 173 



is worth}' of further investigation. It should throw 

 hght on (1) the similarities between twins; (2) the 

 limits of relation between handwriting and character. 



Another important fact which the student of 

 eugenics must needs bear in mind is not only that a 

 vast array of biological character-differences exist, 

 which are being reshuffled from generation to 

 generation, but that in the individual the greater 

 number of these characters will be in the heterozygous 

 or hybrid {i.e., inconstant or splitting) condition. 

 This has both advantages and disadvantages from 

 the point of view of selective mating and offspring. 

 Among the advantages may be reckoned the fact 

 that the offspring are sure to show a variet}^ of mental 

 and physical characteristics — i.e., to differ markedly 

 from each other (except in the case of identical twins) ; 

 while the disadvantage is measured by the number of 

 unfavourable recessive characters carried bv both 

 parents. The fact that both parents may be carrying 

 some of the same unfavourable or deleterious recessive 

 characters is the chief and probably the only sound 

 biological argument against cousin marriages. Here it 

 may be pointed out, that w^hile it is impossible to know 

 with certainty from a human pedigree all the recessive 

 characters which are carried by an individual, yet it 

 will be possible to determine some of them from a study 

 of the ancestors, and particularly of the collateral 

 lines, such as uncles and cousins and their families. 



Eugenic action should, then, be based upon four 

 separate factors: (i) Positive selection for desirable 

 qualities, w^hich are frequently dominant ; (2) negative 

 selection against undesirable recessive qualities which 

 appear in collateral or ancestral lines, and may 

 therefore be carried in the family germ plasm ; 

 (3) isolation of individuals having undesirable 

 dominant qualities ; (4) correlated with this should be 

 an effort to foster matings between individuals 



