The Galaxy

Science fiction writers imagine two kinds of galaxy: humans only or multi-species. James Blish's Cities In Flight (London, 1981) is kind of intermediate. There are a few other species and some of them are significant but they remain off-stage throughout the entire Tetralogy. Thus, we see only human beings interacting with each other.

The nearest we come to an alien appearance is when we hear a voice from the Vegan orbital fort:

" 'PEOPLE OF EARTH. US THE CITY OF SPACES CALLS UPON YOU...YOUR NATURAL MASTERS TO OKAY, THE MANS OF STARS, WHO THE UNIVERSE-UNDERSTANDING LONG-LIFE UNDERSTANDING INHERITORS, THE INFERIOR HOMESTAYING DECADENT EARTH PEOPLES THEREOVER, THE NEW RULERS OF, ARE ABOUT TO BECOMING. US INSTRUCTS YOU SOON TO PREPARE -'" (p. 416)

We are to imagine this spoken in a "...mouthy voice..." (p. 416) "Okay" may be a mispronunciation of "Okie."

The Okies, including those who later become New Earthmen, must contend with some powerful, understated villains:

bindlestiffs;
Vegans;
IMT;
Jorn the Apostle;
the Web of Hercules.

The Vegan orbital fort attacks Earth under cover of the Okies' "March of the Cities" but Amalfi flies a planet in front of it. At the Metagalactic Centre, both the Herculean attack and the Hevian counter-attack are fatal but both sides survive long enough to influence subsequent universes: a stalemate or one-one draw.