Time for a walk! Let’s go to a forest, or even a woodlot near your house and ask, how many different groups of animals will you find there? Not just in plain sight, but underfoot.

Well, says E. O. Wilson, Harvard’s most famous biology professor, in his newest book, Half Earth: Our Planet’s Fight for Life, you’ll see lots and lots of insects, spiders, mites, birds, caterpillars, frogs, millipedes, centipedes, crustaceans, moles, beetles, shrews, snails, slugs, earthworms, and roundworms, not to mention a whole lot of animals you can’t see without a magnifying glass. But altogether, Professor Wilson says, seven different animal groups (phyla) will show up in the forest. Remember that number: seven.

OK, done with the forest. (There’s