Nature inspires both parents with love for their little ones, and this love is not something artificial. It is not just a love fed by reason, like the idea of brotherhood born of an intellectual wish to unite mankind. The love we find in infancy shows what kind of love should reign ideally in the grown-up world: a love able, of its own nature, to inspire sacrifice, the dedication of one ego to another ego, of one's self to the service of others.
In the depth of their love, all parents renounce their own lives to dedicate them to their children. And this devotion is natural to them. It gives them joy and does not feel sacrificial. The efforts parents make for their children give them joy; they are a part of parenthood itself. The child awakens what adults think of as an ideal; the ideal of renunciation, of unselfishness - virtues almost unreachable outside family life.
The fiercest and most savage of beasts turns tame and gentle with its young. How tender are lions and tigers with their cubs; how fierce the timid doe in protecting her dam! It seems as though with all animals, their usual instincts are reversed in the presence of their little ones. It is as if parenthood gave them special instincts, which override the normal.
If survival depended solely on the triumph of the strong, then the species would perish. So the real reason for survival, the principal factor in the "struggle for existence," is the love of adults for their young.