e. From these simple
facts is evolved the intimate relationship which exists between sexual
love and other sentiments of sympathy, that is to say affection, or
love in the more vague and more extended sense of the term.

To every sentiment of sympathy between two individuals (sympathy forms
part of the sentiments of pleasure) there is a corresponding contrary
correlative sentiment of grief, when the object of sympathy dies,
becomes sick, takes flight or is carried off. This sentiment often
takes the form of simple sadness, but it may attain a degree of
incurable melancholy. Among certain monkeys and parrots, we often see
the death of one of the conjoints lead to the refusal of all food and
finally to death of the survivor, after increasing sadness and
depression. Removal of the young produces a profound sadness in the
female ape. But when an animal discovers the cause of the grief, when,
for instance, a stranger attempts to take away his mate or his young,
a mixed reaction of sentiment is produced, that is to say anger or
even fury against the perpetrator of the deed.

Jealousy is only a special form of this anger. The sentiment of anger
and its violent and hostile expression constitute the natural reaction
against one who disturbs a sentiment of pleasure, a reaction which
tends to reestablish the latter. The power of the sentiment of anger
increases with the offensive and defensive faculties, while, in weak
and peaceful beings, terror and sadness to a great extent take their
place. On the other hand, the sight of defenseless prey suffices to
provoke, in the rapacious who are strong and well armed, by simple
reflex association, a cruel sentiment of voluptuous anger, which is
also observed in man.

=Sentiment of Duty.=--Another derivative of the sentiment of sympathy
is that of _duty_, that is the moral sense. All sentiment of love or
sympathy urges the one who loves to certain acts destined to increase
the welfare of the object loved. This is why the mother nourishes her
young and plucks feathers and hairs to make them a soft bed; and why
the father brings food to his wife and young, and defends them against
their enemies. All these acts, which are not to the advantage of the
individual but to the object or objects of his sympathy, exact more or
less laborious efforts, courage in the face of danger, etc. They thus
provoke an internal struggle between the sentiment of sympathy and
egoism, or the unpleasantness of unde