inherited; therefore the make-up of
the race can be changed by any method which will alter the relative
proportions of the contributions which different classes of men make to
the following generation.

For applied eugenics, it is sufficient to know that mental and physical
differences are inherited; the exact manner of inheritance it would be
important to know, but even without a knowledge of the details of the
mechanism of heredity, a program of eugenics is yet wholly feasible.

It is no part of the plan of this book to enter into the details of the
mechanism of heredity, a complicated subject for which the reader can
refer to one of the treatises mentioned in the bibliography at the close
of this volume. It may be worth while, however, to outline in a very
summary way the present status of the question.

As to the details of inheritance, research has progressed in the last
few years far beyond the crude conceptions of a decade ago, when a
primitive form of Mendelism was made to explain everything that
occurred.[43] One can hardly repress a smile at the simplicity of those
early ideas,--though it must be said that some students of eugenics have
not yet outgrown them. In those days it was thought that every visible
character in man (or in any other organism) was represented by some
"determiner" in the germ-plasm; that by suitable matings a breeder could
rid a stream of germ-plasm of almost any determiner he wished; and that
the corresponding unit character would thereupon disappear from the
visible make-up of the individual. Was a family reported as showing a
taint, for instance, hereditary insanity? Then it was asserted that by
the proper series of matings, it was possible to squeeze out of the
germ-plasm the particular concrete _something_ of which insanity was the
visible expression, and have left a family stock that was perfectly
sound and sane.

The minute, meticulous researches of experimental breeders[44] have left
such a view of heredity far behind. Certainly the last word has not been
said; yet the present hypotheses _work_, whenever the conditions are
such as to give a fair chance. The results of these studies have led to
what is called the factorial hypothesis of heredity,[45] according to
which all the visible characters of the adult are produced by (purely
hypothetical) factors in the germ-plasm; it is the factors that are
inherited, and they, under proper conditions for development, produce
the charac