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Wikipedia – plot summary [continued]

Soon, the Once-ler's Thneed-making business expanded tenfold and now used delivery trucks to take out the shipments. The Lorax eventually came back complaining to the Once-ler that the factories were belching out so much "smogulous smoke" that it was giving the Swomee Swans sore throats, leaving them unable to sing. After the Lorax sent them off, he also complained to the Once-ler about his machinery making a goo by-product called "Gluppity Glup" and "Shloppity Shlop," and how it was being dumped into the ponds where the Humming Fish live, leaving them unable to hum and forcing the Lorax to send them away too.

The Once-ler, disgruntled by this, still dismissed the Lorax's pleadings and declared his intention to keep "biggering" his operations, but at that very moment, the "The very last Truffula tree of them all" falls. Without raw materials, his factory shut down; without the factory, his relatives left. Then the Lorax, silently, with one "very sad, sad backward glance", lifted himself by the seat of his pants and flew away through the clouds.

The Once-ler lingered on in his crumbling residence, living in seclusion and remorse, while pondering over a message the Lorax left behind: a stone slab etched with the word "Unless". In the present, he now realizes what the Lorax meant. He tells the boy, "Unless someone like you cares a whole awful lot, nothing is going to get better. It's not." The Once-ler then gives the boy the last Truffula seed and tells him to plant it, saying that if the boy grows a whole forest of the trees and keeps them protected from logging, "the Lorax, and all of his friends may come back."