In animals somewhat higher in the creative scale, more or less powerful feelings of affection may develop out of their sex association. There is affection on the part of the male for his mate, and on the part of the female for her young. Often these feelings develop into a strong, lasting affection between the sexes, and years of what might be called faithful matrimonial union have been observed in the case of birds. This in itself is sufficient to establish the intimate relationship between love in a sex sense and love in a general sense. And even in the animal creation we find the same analogy existing between these feelings of sympathy and their opposites which occur in the case of human beings. Every feeling of attachment or sympathy existing between two individuals has a counterpart in an opposite feeling of discontent when the object of the love or attachment in question dies, falls sick, or runs away. This feeling of discontent may assume the form of a sorrow ending in lasting melancholy. In the case of apes and of certain parrots, it has been noticed that the death of a mate has frequently led the survivor to refuse nourishment, and die in turn from increasing grief and depression. If, on the other hand, an animal discovers the cause of the grief or loss which threatens it; if some enemy creature tries to rob it of its mate or little ones, the mixed reactive feeling of rage or anger is born in it, anger against the originator of its discontent. Jealousy is only a definite special form of this anger reaction.
A further development of the feeling of sympathy is that of duty. Every feeling of love or sympathy urges those who feel it to do certain things which will benefit the object of that love. A mother will feed her young, bed them down comfortably, caress them; a father will bring nourishment to the mother and her brood, and protect them against foes. All these actions, not performed to benefit the creature itself, but to help its beloved mate, represent exertion, trouble, the overcoming of danger, and lead to a struggle between egoism and the feeling of sympathy. Out of this struggle is born a third feeling, that of responsibility and conscience. Thus the elements of the human social feelings are already quite pronounced in the case of many animals, including those of love as well as sex.