Disputes and reorderings of much the same type can be found in all the other realms of the living, so keeping an overall tally is not nearly as straightforward a matter as you might suppose. In consequence, the rather amazing fact is that we don't have the faintest idea — "not even to the nearest order of magnitude," in the words of Edward O. Wilson — of the number of things that live on our planet. Estimates range from 3 million to 200 million. More extraordinary still, according to a report in the Economist, as much as 97 percent of the world's plant and animal species may still await discovery.

Of the organisms that we do know about, more than 99 in 100 are only sketchily described — "a scientific name, a handful of specimens in a museum, and a few scraps of description in scientific journals" is how Wilson describes the state of our knowledge. In The Diversity of Life, he estimated the number of known species of all types — plants, insects, microbes, algae, everything — at 1.4 million, but added that that was just a guess. Other authorities have put the number of known species slightly higher, at around 1.5 million to 1.8 million, but there is no central registry of these things, so nowhere to check numbers. In short, the remarkable position we find ourselves in is that we don't actually know what we actually know.