travelling in straight lines through the galaxy, from planet to planet
While Trek often gives the impression that every star in the galaxy has a planetary system containing at least one duplicate Earth, the truth might be that suitable worlds are still few and far in between. Perhaps a "straight line" means the path formed by neighboring suitable worlds, and such paths meander through an otherwise barren galaxy in a fashion that results in at least short stretches that can be interpreted as more or less straight?
It would be possible to predict that a menace that came from Alpha via Beta and Gamma will next move to Delta, not because this is literally a beeline, but because the closest (and only) star system that features a Class M planet in the hemisphere of sky opposite from Beta as viewed from Gamma is Delta. That is, there's a path drawn in the sky already, and the monster isn't creating it, but making apparent use of it.
Both episodes catch up with their respective antagonists after they'd been doing their thing for at least centuries, so it seems possible that there is more to their travels than just plain travelling. I'm not saying the two are connected with each other, just that their stories are long ones about which we only know of the here and now of it.
Agreed. The DDM is no doubt a similar case; the Space Amoeba, likewise. These creatures move from star to star, not doubling back, and only first being observed when they enter the patrol area of a Starfleet cruiser.
But that may well be observer bias. Other civilizations have no doubt encountered these creatures in a similar manner, seeing how they appear from the depths of space and seem to mercilessly head for the center of the local civilization. But that's not because the creature would have a goal. It's because the civilization stands in its way. When it passes through, it creates the impression it's hell-bent on harming the civilization - but every civilization in its path gets that mistaken impression.
If these things had a goal
and started out recently, say, at historical Earth, they would probably be going faster and faster as they learn, or grow impatient, or whatever. Or they might slow down. A steady pace, even if it includes regular stops, might suggest a much longer path of which our heroes only recognize a short stretch.
Our heroes terminate a great many such journeys! Is that an exception, or is the galaxy simply so vast that the monsters can roam for centuries even when they are eminently stoppable by primitives such as 23rd century humans? The Federation might be quite porous to invaders in that century still...
Timo Saloniemi