For some information on the Buddhist monk and teacher Han Shan Te Ching (1546–1623 CE) see this post.

Put a fish on land and he will remember the ocean until he dies. Put a bird in a cage, yet he will not forget the sky. Each remains homesick for his true home, the place where his nature has decreed that he should be. 

Man is born in the state of innocence. His original nature is love and grace and purity. Yet he emigrates so casually without even a thought of his old home. Is this not sadder than the fishes and the birds?