The Aquarium is gone. Everywhere, giant finned cars nose forward like fish;
Lowell circles back to the beginning mentioning the long-gone aquarium. This also snaps us back into the present day.
The aquarium seems to have been replaced by a world much less appealing to the speaker. The cars are creeping around like fish. The world, it seems, has been turned upside down. We're getting that things have gotten worse feeling again.
It also seems sort of space-agey. Just two stanzas ago, Lowell wrote, "Space is nearer." He's showing us an unfamiliar, strange-looking future, filled with "giant finned cars."
a savage servility slides by on grease.
Vocab alert! Servility can mean: 1. having or showing excessive willingness to serve or please others; 2. of or characteristic of a slave or slaves. Do we think Lowell used this word accidentally in a poem in part about the continued oppression of African Americans? Of course not!
Okay, so now he's wrapping it all in one bundle: the fish, the progress (technology, city, cars), and the freed slave soldiers. How exactly they fit takes some careful review.
Savage means fierce, violent, or uncontrolled. Lowell is doing it again: pairing unlikely or even opposite adjectives ("savage" with "servility") to create a more powerful meaning. So, savage + servility = someone (or something) that wants to be wild and strong but has been harnessed into servility somehow. Hmm.
Now how can we apply this to the poem?
Well, the infantrymen fought with savage ferocity for their country and for their own civil liberties. They were servile, however, because they still didn't have nearly any of the civil rights that white people did.
What Lowell has been hedging at, and is finally saying in the end, is that it was messed up for these guys to sacrifice their lives for a country that didn't really value them enough, and as he continues through the poem, he shows how little progress has been made in terms of civil rights and equality.
These final two lines aren't talking about the soldiers directly, but Lowell is saying that, even though the Civil War chapter is over, there hasn't been enough change or progress. It's not something as a country that we've been washed of—thus the slides by on grease (grease is dirty).
Now, keep in mind when this poem was written! The poem came out in the early 1960s—the height of the Civil Rights movement. This poem charts how little progress had been made between the time when the Union soldiers died and the time the poem was being written. Lowell includes the other aspects of the poem—the aquarium, the construction in Boston—to paint a larger picture for the reader. The fish were trapped, enslaved in a way. That's the way this awful world works, Lowell is saying, and—despite decades of history—it hasn't really changed. Bummer.