Tracing the attribute of Love to its source in the parental function, it becomes clear that this function cannot be dismissed thus in a phrase.
There are two parents. And the parts played by these, respectively, not only differ widely in their nature, but they are signally disproportionate in their share of the labours involved. For while the male bears the brunt of the struggle with environment, for his own and for survival of his mate and offspring, upon the female falls the biological stress of pregnancy and lactation, and the material cares of upbringing.
The reproductive function of the male is but slight and cursory. With the female lies the tax of havening the embryo before birth, of nurturing it with her blood and substance, of suffering the drain it makes upon her vital energy, the burden of its weight; with, finally, the anguish and the dangers of delivery. And having come through all this, the subconscious and involuntary sacrifice is replaced by further--but now voluntary sacrifices. She not only continues to feed it with her living substance, but she employs brain and wit and bodily effort in tending, safeguarding and rearing it.
Meanwhile the sire--among the lower creatures, at all events--detaches himself with lordly indifference from any portion in these later, as he went free of the earlier obligations. He shares his prey with her and with their young. He defends them from the natural enemies of all. Sometimes he condescends to play for minutes with his cubs. But excepting among birds, the male parent takes little or no part in the upbringing of his family.
As with Love, so with Fatherhood, we take it as matter-of-course that this sprang and has evolved to present developments directly out of natural instinct. But as Love did not evolve out of the sex-instinct, neither did father-love evolve from a paternal instinct inherent in the lower animals and in primal man.
"The world was now beginning to fill with Mothers, but there were no Fathers, ... while Nature has succeeded in moulding a human Mother and a human child, he still wanders in the forest, a savage and unblessed soul.
"This time for him is not lost. In his own way he also is at school, and learning lessons which will one day be equally needed by humanity. The acquisitions of the manly life are as necessary to human character as the virtues which gather their sweetness by the cradle; and these robuster elements--strength, courage, manliness, endurance, self-reliance--could only have been secured away from domestic cares.... The Evolution of a Father is not so beautiful a process as the Evolution of a Mother, but it was almost as formidable a problem to attack.... If Maternity was at a feeble level in the lower reaches of Nature, Paternity was non-existent.... When we leave the Birds and pass on to the Mammals, the Fathers are nearly all backsliders. Many are not only indifferent to their young, but hostile; and among the Carnivora the Mothers have frequently to hide their little ones in case the father eats them."