Richard Payton Jr., age 7, of Petersburg, Virginia, for his question:

Are there more land or water animals?

Suppose we tried to count them one by one. Let's start with the herring that live in the sea and the mice that live on the land. Most likely we could count more herring than mice. All the herring are much alike, but there are dozens of different varieties of mice. Besides, trying to count every living creature one by one would take much too long.

At our our scientists have found most of the different varieties of animals, but not all of them. After all, some of them are so small that it takes thousands of them to measure one inch. We have found just about all the big ones and most of the medium sized ones. But every year scientists discover many more kinds of insects for example. So we cannot give the exact number of the different animals living on land and in the sea. But we can make a fairly good guess. To do this, we count up the ones we already know and allow for those that are still waiting to be discovered. This sort of guess is called an estimate.

Altogether, the number of different animals is estimated to be about three million. And, of course, some of these assorted animals live in the sea and some live on the land. Only about one quarter of the earth is dry land. So you would expect that most varieties of animals live in the sea. But this is not so. The scientists are not guessing about this. They are sure that far more different kinds of animals enjoy life on dry land.

When you think of life in the sea, naturally you think of the scaly, bony fishes    about 20,000 kinds. The land has 25,000 different birds alone    5,000 more than all the big and little fishes. The sea has about 50,000 assorted shellfish    shrimp and lobsters, clams and crabs. But the land has about 300,000 different beetles, more than 100,000 assorted moths and butterflies, more than 100,000 ants, wasps and bees. Altogether, there are more than a million varieties of land dwelling insects  more insects than all the different creatures of the sea. This is not counting the birds, the reptiles or the mammals which also inhabit the land.

Naturally, there are reasons why so many more kinds of animals live on land, even though the sea is so much bigger. For one thing, all animals need oxygen. The  with sea dwellers take oxygen dissolved in the water. But there is more breathable oxygen in the air. Also, there are not as many varying environments in the sea. The dry land has more than a million different places to live. It has cold, warm and hot regions, dry deserts and swamps, high mountains, prairies and forests. And each place provides a different type of home for many varieties of animals and insects.

Scientists tell us that the land did not always have most of the animals. Once upon a time, all of them lived in the sea. About 400 million years ago, a few of them left the water. They learned to breathe air, and they thrived and multiplied. In time they spread out to different places and far outnumbered the different animals left behind in the sea.