A contract investigator who tracks down crimes against endangered species finds a mammoth tusk that’s only 30 years old according to radiocarbon dating.

I didn’t understand the logic of this short story, which is part of Nature’s Futures series of short, short sf stories. The grandfather, Hubert, is traveling forward in time, begging his grandson to kill him so that he won’t invent a time machine that he’s already invented—but I can’t see how killing him after the fact will do any good. Please explain it to me!

In any case, thank you to the kind librarian at the Norlin Library who made an electronic copy for me when we couldn’ttrack down a hard copy of the journal.