What is the meaning of spring eternal?

You fear lest the dismal prophesies of world disaster, of cataclysm, of the destruction of half the human race, which you hear from many sources may tend to discourage the world. 

 Remember that hope springs eternal in the human breast. 

And if the minds of men are familiar with the idea of cataclysm, they will more readily adjust themselves to lesser changes. 

 Read the Old Testament. 

 The most dismal prophecies were not verified, but changes came.

—The Judge [Last Letters from a Living Dead Man]