ABSTRACT
The most obvious differences between different animals are differences of size, but for some reason the zoologists have paid singularly little attention to them. In a large textbook of zoology author find no indication that the eagle is larger than the sparrow or the hippopotamus bigger than the hare, though some grudging admissions are made in the case of the mouse and the whale. The higher animals are not larger than the lower because they are more complicated. They are more complicated because they are larger. Just the same is true of plants. The simplest plants, such as the green algae growing in stagnant water or on the bark of trees, are mere round cells. Large animals on the other hand only require relatively small eyes, and those of the whale and elephant are little larger than our own. Such are a very few of the considerations which show that for every type of animal there is an optimum size.
