never occurs to him that this situation could change and that, in a pitiless system, once he is weakened, there will a dozen stronger men to take his place.

This reality comes home to him only after he injures his arm and realizes he will not be able to find work. The following quote shows his dawning awareness of what this world of competition will do to him:

He was like a wounded animal in the forest; he was forced to compete with his enemies upon unequal terms. There would be no consideration for him because of his weakness—it was no one's business to help him in such distress, to make the fight the least bit easier for him.

Jurgis finds that even the world of begging (to which he is reduced) is a fierce, competitive battle. He discovers that he is

a blundering amateur in competition with organized and scientific professionalism.

Jurgis comes to understand that the deck is stacked against people like him:

That was “competition,” so far as it concerned the wage-earner, the man who had only his labor to sell; to those on top, the exploiters, it appeared very differently, of course—there were few of them, and they could combine and dominate, and their power would be unbreakable. And so all over the world two classes were forming, with an unbridged chasm between them—the capitalist class, with its enormous fortunes, and the proletariat, bound into slavery by unseen chains.