he more frequent
1:2:1 or 3:1 are explainable upon essentially the same relations as
these simpler and more typical ratios. And further, many less usual
Mendelian phenomena, which we cannot undertake to describe here,
are associated with what the specialist technically terms "sex
limitation," "gametic coupling," and the like.

It is often said that the Mendelian formula has a very limited
applicability to human heredity. This is probably true if we consider
carefully the grammatical tense in which this statement is made. And
yet it is almost certainly true that heredity in man is to be
described by this law. This apparent paradox is easily explained. The
only characters whose history in heredity follows this formula are the
unit characters. A complex trait is not heritable, as a whole, but its
components behave in heredity as the separate units. It is perfectly
well known that we are deeply ignorant regarding this phase of human
structure. Our ignorance here is not the necessary kind, however, it
is merely due to the newness of the subject--we have not had time to
find out. How can we say that a complex trait is or is not inherited
according to some form of Mendel's law when we do not know the nature
of the units of which it is composed? We can make no statements about
the Mendelian inheritance of such a trait until it is factored into
its units. A considerable number of human characteristics are really
known to be heritable according to this formula, enough so that
several general rules of human heredity have been formulated. But it
is also quite within the range of possibility that some traits really
do not follow this law, although it cannot yet be said definitely
that this is or is not the case. On the whole, then, we cannot, for
the next few years, expect too much from the application of Mendel's
laws to human heredity, however much this is to be regretted.

Shall we then decline to say anything about the heredity of the great
bulk of human characteristics? By no means: we have seen that in our
bagatelle board we talk very definitely about the distribution of all
the peas, though only about the probable history of one pea. Mendel's
law deals with individual inheritance. When we cannot apply this
formula we have left still the possibility of talking about human
heredity in the group as a whole. That is to say, we have left the
opportunity of describing heredity by the statistical methods, with
the crowd, not the i