The first two sentences in the novel that speak specifically about selection for work in the stockyards are in the second paragraph of the book's second chapter:
That was why he had been picked out on one important occasion; for Jurgis had stood outside of Brown and Company's “Central Time Station” not more than half an hour, the second day of his arrival in Chicago, before he had been beckoned by one of the bosses. Of this he was very proud, and it made him more disposed than ever to laugh at the pessimists.
These lines should be understood within the context of what is going on in the novel. Jurgis is young and very strong. He is proud to be chosen so quickly for the stockyard work because it shows he is winning in the competition to get ahead. He is glad at this point to be part of a system that chooses the strong and discards the weak. He scorns and blames the men who don't get chosen as being unworthy. He feels like a winner.
The "pessimists" try to tell Jurgis that one day it will be him standing around for months on end, unable to be hired, but he simply can't believe that he will ever not be at the top of his form.
Sinclair's purpose is to show how the bosses put the men in competition with each other. Already, he is strongly foreshadowing that a time will come—and it will in the novel, and quickly—that Jurgis will be too broken down to find work. But by that time, it will be too late for him—there will be many other fresh young "Jurgises" looking for work that the system will replace him with. They too will be oblivious to how short the time will be before they are run into the ground, used up, and discarded.
Sinclair wants to make the point that the only hope and security for these men is to band together in common cause and demand decent wages, working conditions, and job security. Individualism doesn't work: the system too quickly grinds people up. Nobody can "win" this competition because nobody can stay fresh and strong for long: the only solution is collective action. Young men like Jurgis at first may think they are subsidizing the weak and broken to join them in solidarity, but they will soon be the very people they now despise.