Whew. The long stroll down memory lane is over. The Once-ler tells the boy (remember him?) that ever since the Lorax left, he's been desperately trying to figure out what the Lorax meant by UNLESS (shouted, no less!).
Until the boy came and asked about the Lorax, the Once-ler couldn't figure it out. But now it's "perfectly clear" (247):
He tells the boy, "UNLESS someone like you/ cares an awful lot,/ nothing is going to get better./ It's not" (248-251).
With that, the Once-ler drops something down to the boy: the very last Truffula Seed in existence.
The Once-ler's last words to the boy: "Grow a forest. Protect it from axes that hack./ Then the Lorax/ and all of his friends/ may come back" (264).