nalyzable 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.

It is thus impossible to tell as yet with any security or precision
the biological components of any single mental trait.
The evidence at our disposal, however, does confirm us in the
belief that one of the most significant and certain causes of
individual differences, whether physical or mental, is immediate
ancestry or family. Individuals are made by what they
are initially, and, as we shall presently see, therefore largely
by their inheritance. With the latter, environment can do
just so much, and no more. And the most significant and
effective part of an individual's inheritance is his family for
some generations back, rather than the race to which he belongs.

THE INFLUENCE OF THE ENVIRONMENT. Those