and possesses the traits that are common to members of that
stock; and the same with family traits. The criterion of universality,
in the light of these facts, comes down to this: that _when all
individuals having the same descent show a trait in common, that
trait is to be regarded as belonging to their native
constitution--unless evidence can be brought forward to the contrary_.

Smoking is universal among many Malay peoples, but we know, as a
historical fact, that it was introduced among them after the discovery
of America, not very many generations ago. Superstition is universal
among some peoples, but we see the superstitious beliefs and practices
taught by the older to the younger generation. Similarly with any
specific language. It may very well be true in such cases that the
universal practice appeals to some native tendency of the people; but
the specific practice is handed down by tradition and not by
inheritance.

Some Native Traits Are Far from Being Universal

Though the universality of a trait creates a certain presumption in
favor of its being native, the opposite is not always true, for a
trait may be native and yet appear in only a fraction of those who
have a common descent. Eye color is certainly native, and yet one of
two brothers may have blue eyes and the other brown. Mental deficiency
runs in families, but usually some members of such families have {99}
normal mentality. Genius is almost certainly a native trait, but it is
the reverse of universal. The fact is that, along with certain traits
that appear in all, the native constitution of a stock provides also
for traits that appear only sporadically. Enough has been said to show
that the criterion of universality is one that needs to be applied
with judgment.

Why Acquired Traits Differ from One Individual to Another

Acquired traits are on the whole much less universal, much more
individual, than native traits. They are readjustments of the
individual to environmental conditions; and, as the environment
varies, so the adjustments vary, even when native traits are the same.
Acquired traits are often specializations of the native traits, as any
specific language is a specialization of the vocal utterances that are
native and common to all men, and as the peculiar gait of an
individual is a specialization of the universal walking movement. The
gait differs with the environmental differences to which the
individual has adapted himself, and