The principle of similar structures was first enunciated by Galileo. Applying it mechanically, it appears that the strength of a structure varies as the square of the linear dimensions, and the weight as the cube. This principle and its mechanical applications have been widely illustrated by reference to the bodies of animals, and many of their peculiarities are necessary consequences of its effect in the course of their evolution. For instance, the columnar legs of the elephant or of the moa are held to be the inevitable sequel to the large size and consequent weight of those animals, while the thin arched legs of insects are only possible where the body itself is small and light.