It is difficult to be sure, but we might guess that they are becoming increasingly rare.

These stones have become increasingly rare and are difficult to find.

The shop is one of those places, increasingly rare in this city, that time seems to have passed by.

By 1998, the sketches became increasingly rare on the show.

Over the centuries, the play has become increasingly rare on stage.

Winter movements south of their range, however, appear to have become increasingly rare.

A fight between two papers is an increasingly rare sight in America.

The increasingly rare term short subject means approximately the same thing.

In the fall of 2001, the dream team was an increasingly rare sight.

The old idea of an essay with a beginning, a middle, and an end is increasingly rare.