Of all the things that distinguish man from the rest of the animal kingdom, the most important is his brain. Many of the lower animals have no brain at all, or a tiny one, or one that is poorly developed. For instance, an earthworm has a brain about the size of a pinhead, a rabbit has a thimble-sized brain. The brain of a man weighs, on the average, about 1.3 kilograms.

By the way, the size of the brain is not the most important thing about it. An elephant has a bigger brain than man, but it is not as well developed.