Souls do their best to continue down the path of least resistance, to continue doing what it knows. Meaning, they almost always reincarnate into the same species, around other Souls they’re familiar with.
This isn’t a steadfast rule (Mizar is a cat once, Toby is from demon lineage), but it’s extremely rare to see this being deviated from.
One thing that does not happen though, is Souls hopping worlds. They can’t cross dimensions, and they don’t cross an enormous amount of space between viable hosts. This includes mental space – meaning they couldn’t hop between two habitable planets right next to each other, if both worlds aren’t aware of one another. But they could cross the entire galaxy, a hundred thousand light-years wide, if two far-off societies knew of each other’s existence.
So, Souls on earth won’t be able to reincarnate onto other planets, into sentient life, until a decent portion of life on earth knows of another world with similarly sentient life.
