Animals are different. An elephant differs from a mouse, both in shape and size. Size is one of the most important aspects of an animal's endowment, and yet size differences are so obvious that often we give no further thought to them. We know that the elephant is much bigger than the mouse, but we rarely think about how much bigger; in fact, an elephant weighs 100 000 times more than a mouse. The smallest shrew, when fully grown, is only one-tenth the size of a mouse, or one-millionth the size of an elephant.

The world we live in is governed by the laws of chemistry and physics, and animals must live within the bounds set by those laws. We shall see that body size has profound consequences for structure and function and that the size of an organism is of crucial importance to the question of how it manages to survive.

Let us take a closer look at the enormous size differences among living organisms (Table 1.1). Each single step in this table represents a 1000-fold difference in size, and the total difference between the smallest and largest organism listed is 1021. The blue whale, which may exceed 100 tons, is the largest living animal, but the giant sequoia trees of California outweigh the largest whales by 10-or 15-fold.