From this height it was as if the Chums, who, out on adventures past, had often witnessed the vast herds of cattle adrift in ever-changing cloudlike patterns across the Western plains, here saw that unshaped freedom being rationalized into movement only in straight lines and at right angles and a progressive reduction of choices, until the final turn through the final gate that led to the killing floor.
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Sure, we get a depressing look at the meatpacking industry here. But we also get a nice metaphor for people moving from rural life into the city.
I wrote this in my recent Gravity's Rainbow entry but since it concerned this line, I'm just going to copy and paste it here too!
In postmodern writing, you encounter a lot of images and metaphors of the Labyrinth. In these two passages [This was the other passage: "Something was done to him, and it may be that Katje knows what. Hasn't he, in her "futureless look," found some link to his own past, something that connects them closely as lovers? He sees her standing at the end of a passage in her life, without any next step to take—all her bets are in, she has only the tedium now of being knocked from one room to the next, a sequence of numbered rooms whose numbers do not matter, till inertia brings her to the last. That's all."], you get the exact opposite. You have people (and cows! But I think the cows really represent the immigrants of Chicago here! (hell, they really just represent us all and the aging process)) whose lives, while starting out full of freedom and choices, have come to that point where the previous choices made have cut off opportunities for further choice, winnowing their lives down to a single path which they can only follow helplessly until death. The cows have been forced into this kind of life just like the people of Chicago (earlier, the city had been described as being a Cartesian grid which the right angles and straight lines of the cow's Stockyard enclosure echo); life on the plains was a kind of freedom while life in the city a trudging journey along a single corridor maze to the killing room floor. Katje has sort of arrived at the same kind of place even though she's still young. But her choices have narrowed her chances at new opportunities faster than most: Blicero's sex slave, Allied spy, obligation to Prentice for rescuing her, Pointsman's tool, and now Slothrop's keeper. Not much choice for her in most of that. Just a roulette ball skipping about from number to number on a circular path, the number where it will eventually come to rest having no real meaning to the journey.
So here Pynchon uses the Chicago Stockyard as a metaphor for life, especially that of the industrial revolution's effect on rural life and the mass immigration to urban centers. In Gravity's Rainbow, he uses the Roulette ball bouncing around a roulette wheel as the metaphor. Apparently anything that describes movement without purpose can be used as a metaphor for life. Like a pinwheel in a graveyard!