It's interesting, but it seems that cars gradually change between boxy and bubbly and back again. Always. There's never a fair mixture of the two styles. One always dominates, and in transitional periods it's hard to tell whether the angles are hard or soft.

Look at some of the first cars. All very, very rough on the edges.

The Box-Bubble pattern.

This continued for a while until they started to round off, still keeping an overall boxy shape in dominant areas...

The Box-Bubble pattern.

And then they went full bubble.

The Box-Bubble pattern.

But boxiness eased its way in....

The Box-Bubble pattern.

And the pattern continued as they became boxy. Some intervals took longer than others, like this boxy phase, but still.

The Box-Bubble pattern.

Really, I don't think they went back to being bubbly again until the '90s.

The Box-Bubble pattern.

The Box-Bubble pattern.

As abruptly as they faded in, the bubbly cars began to go away again.

The Box-Bubble pattern.

The Box-Bubble pattern.

The Box-Bubble pattern.

Now everything is angry and rigid and sharp again.

I'm sure you all could come up with a billion counterpoints, but I do think there's a trend here.