In each period t there are new nodes and edges that join the network while the old ones disappear.

But this is where the other ones that were last tracked disappeared as well.

The good ones probably disappeared over the years.

But as new species appear, so old ones disappear.

Instead of new worlds coming into view, old ones would constantly be disappearing over the horizon, lost from view forever.

"All we can say for sure is that the old ones suddenly up and disappeared."

The small ones disappeared the way they had come.

New jobs are certainly being created, but the old ones are disappearing rapidly.

Truffles occur naturally, but the most prized ones have been disappearing since the late 19th century.

"Seeing new missions appear and old ones disappear is part of the world we live in," she said.