Mental inheritance, as well as physical, is, then, organized in detail. It is not the inheritance of gross total natures, but of particular "mental traits." If we had sufficient data, we should be able to analyze out the unit characters of an individual's mental equipment, so as to be able to predict with some accuracy the mental inheritance of the children of any two parents. In the case of physical inheritance, the laws of the hereditary transmission of any given traits are known in considerable detail. The detailed quantitative investigations of inheritance, following the general lines set by Mendel, have given striking results. Physical traits have been found to be analyzable into unit-characters (that is, traits hereditarily transmitted as units), such as "curliness of hair," "blue eyes," and the like. Mental traits, however, do not seem analyzable into the fixed unit-characters prescribed by the Mendelian laws of inheritance. The success which breeders have had in the control of the reproduction of plants and animals, in the perpetuation of a stock of desirable characteristics and the elimination of the undesirable, has given rise to a somewhat analogous ideal in human reproduction. That eugenics has at least its theoretical possibilities with regard to physical traits, few biologists will question. However difficult it may be in practice to regulate human matings on the exclusive basis of the kind of offspring desired, it is a genuine biological possibility. In a negative way, it has already in part been initiated in the prevention of the marriage of some extreme types of the physically unfit, by the so-called eugenic marriage laws in some states in this country.[1] [Footnote 1: There have been laws, as there is a fairly decided public opinion, adverse to reproduction by the feeble-minded and the morally defective. But (see Richardson: _The Etiology of Arrested Mental Development_, p. 9) there have been a number of cases of feeble-minded parents producing normal children.] But whether scientific regulation of marriages for the production of eugenic offspring is feasible, even apart from the personal and emotional questions involved, is open to question. No mental trait such as vivacity, musical ability, mathematical talent, or artistic sense, has been analyzed into such definitely transmissible unit-characters as "blue eyes" and "curliness of hair." So many unit-characters seem to be involved in any single mental trait that it will be long before a complete analysis of the hereditary invariable determinants of any single trait can be made.