ing a continuous background of feeling to our experience.
Emotions (and moods) are often characterized as being either
positive or negative. Generally speaking, positive in this context refers
to states that have an associated pleasant, enjoyable, or rewarding
subjective experience, while negative refers to states that are associ-
ated with upset or distress.
Charles Darwin was a pioneer in the science of emotion. His book on
The Expression of the Emotions in Man and Animals was published in
1872, thirteen years after On the Origin of Species. Darwin put emo-
tions into an evolutionary context, arguing that emotions are not
unique to humans but evolved as adaptive behaviors that we share
with other animals. With keen observation and insight, he studied the
behaviors of nonhuman animals and made connections between ani-
mal expressions and those of humans (Fig. 21.1).