There is something in natural affection which will lead it on to eternal love more easily than natural appetite could be led on. But there's also something in it which makes it easier to stop at the natural level and mistake it for the heavenly. Brass is mistaken for gold more easily than clay is. And if it finally refuses conversion its corruption will be worse than the corruption of what ye call the lower passions. It is a stronger angel, and therefor, when it falls, a fiercer devil.