From the ending to the 1972 cartoon, showing that things are not hopeless. With the way the line is delivered and the swelling music, really adds even more weight to this line:
The Once-Ler:"You're in charge of the last of the Truffula Seeds. And Truffula Trees are what everyone needs. Plant a new Truffula. Treat it with care. Give it clean water. And feed it fresh air. Grow a forest. Protect it from axes that hack. Then the Lorax and all of his friends may come back."