Addender

The aliens will live in different regions of the same planets. Thus will coexistence flourish, and when plagues break out, each species will bring in doctors and nurses from an immune species.

Addendum

The same won't be true of the other planets we will undoubtedly colonize because far smaller regions will be inhabited. When you can pick and choose the best places, you don't want to leave anyway.

Earth's Future

Canals everywhere. Every continent divided into even grids by canals. Cars: Obsolete. Trains: Obsolete. Planes: Rigged for water landings. Bicycles: Retained for health reasons only. Motorcycles: I'm pretty sure they're used to annoy everybody even today, so nothing will change there.

An Alternate Earth

What if the land were sea and the sea land? Not enough water, for one thing. The Panama Bridge would still be pretty good.