In the lower animals with no separate sexes egoism reigns absolutely. Each individual eats as much as it wants, then divides, buds or conjugates, thus fulfilling the sole object of its existence. The same principle holds in the lower stages of reproduction by separate sexes. Spiders give us a good example. In these, copulation is a dangerous act for the male, for if he is not extremely careful he is devoured by the female, sometimes even before having attained his object, often soon afterward, in order that nothing may be lost. However, the female shows a certain consideration for her eggs, and sometimes even for the young after they are hatched.
In higher stages of the animal kingdom sentiments of sympathy may be observed, derived from the sexual union of individuals. These are sentiments of attachment of the male for the female, and especially of the female (sometimes the male also) for their progeny.
Such sentiments become developed and may be transformed into intense love between the sexes, of long duration. Birds, for instance, often remain faithful for many years, and even for life. 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 reëstablish 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.