I recently came across the idea that uniformitarianism (u'ism) is the opposite end of the same concept as catastrophism (c'ism).

The link is scale. Swatting a fly is a catastrophe for the fly, but not for a human. My father died when I was 14 which was a catastrophe for me but not for the people in the county where I lived.

Meteorites hit the earth all the time, but seldom do damage. However, a large
asteroid is just a very large meteorite and could do a lot of damage. A much larger body could do a lot worse still.

U'ism has no problem with bank or cliff erosion destroying the creatures which
inhabit them. Or with prolonged drought shriveling vegetation, or causing forest fires. Or with wet weather rotting seeds.

There is plenty of 'junk' in space, much coming close to earth. One object last year came within the Moon's orbit. The Moon and some planets show signs of continual bombardment. Is that u'ism if it misses us but c'ism if it hits?

I would welcome your views.

The big question is whether the ocassional cosmic disaster is all-important in deciding the course of evolution on Earth, or evolution simply proceeds at a steady rate without requiring catastrophic shake-ups.

If you want to blur the two definitions into each other - why not say that, since impacts are a natural process of the Solar System, they are also a routine influence in the ongoing process of evolution? A selection pressure imposed on organisms by their environment, just like forest fires, droughts, volcanos etc... except that impacts are less common, and can have more severe effects when they do happen?

If the history of life since the last major impact event were re-run, the biota on Earth today might come out very different - just due to the randomness of ongoing selection pressures and competition between species. Likewise, if the impact hadn't happened, evolution would again have turned out differently. I reckon uniformitarianism can incorporate catastrophism.

"The big question is whether the ocassional cosmic disaster is all-important in deciding the course of evolution on Earth, or evolution simply proceeds at a steady rate without requiring catastrophic shake-ups. "

There have been several occasions (inc the dinosaurs) when 60-90% (approx) of the species on this planet have been wiped out by major impacts. That looks pretty important.

"I reckon uniformitarianism can incorporate catastrophism."

I would agree with that. They are not mutually exclusive.