Reason’s function in defining the ideal is in principle extremely simple, although all time and all existence would have to be gathered in before the applications of that principle could be exhausted. A better example of its essential working could hardly be found than one which Darwin gives to illustrate the natural origin of moral sense. A swallow, impelled by migratory instincts to leave a nest full of unfledged young, would endure a moral conflict. The more lasting impulse, memory being assumed, would prompt a moral judgment when it emerged again after being momentarily obscured by an intermittent passion. “While the mother bird is feeding or brooding over her nestlings, the maternal instinct is probably stronger than the migratory; but the instinct which is more persistent gains the victory, and at last, at a moment when her young ones are not in sight, she takes flight and deserts them. When arrived at the end of her long journey, and the migratory instinct ceases to act, what an agony of remorse each bird would feel if, from being endowed with great mental activity, she could not prevent the image continually passing before her mind of her young ones perishing in the bleak north from cold and hunger.” She would doubtless upbraid herself, like any sinner, for a senseless perfidy to her own dearest good. The perfidy, however, was not wholly senseless, because the forgotten instinct
- Descent of Man, chapter iii.