"Worm's team estimated the total number of genera, families, orders, classes, and phyla—a designation above class—in each kingdom. That's a relatively easy task

"Worm's team estimated the total number of genera, families, orders, classes, and phyla—a designation above class—in each kingdom. That's a relatively easy task, since the number of new examples in these categories has leveled off in recent decades. By contrast, the number of newly discovered species continues to rise sharply. Using complex statistics, Worm and colleagues used the number of genera, families, and so on to predict Earth's number of unknown species, and their calculations gave them a number: 8.7 million."