According to the University of Chicago paleontologist David Raup,
the background rate of extinction on Earth throughout biological history has been one species lost every four years on average.
According to one recent calculation, human-caused extinction now may be running as much as 120,000 times that level.
In the mid-1990s, the Australian naturalist Tim Flannery, now head of the South Australian Museum in Adelaide,
became struck by how little we seemed to know about many extinctions, including relatively recent ones.
Wherever you looked, there seemed to be gaps in the records—pieces missing, as with the dodo, or not recorded at all,
he told me when I met him in Melbourne a year or so ago.
Flannery recruited his friend Peter Schouten, an artist and fellow Australian,
and together they embarked on a slightly obsessive quest to scour the world's major collections to find out what was lost, what was left, and what had never been known at all.
They spent four years picking through old skins, musty specimens, old drawings, and written descriptions— whatever was available.
Schouten made life-sized paintings of every animal they could reasonably re-create, and Flannery wrote the words.
The result was an extraordinary book called A Gap in Nature, constituting the most complete—and, it must be said, moving— catalog of animal extinctions from the last three hundred years.
For some animals, records were good, but nobody had done anything much with them, sometimes for years, sometimes forever.
Steller's sea cow, a walrus-like creature related to the dugong, was one of the last really big animals to go extinct.