Page:Popular Science Monthly Volume 17.djvu/245

Rh motion of the jaw prevails in the rodents. Other mammals have the lateral motion in a great degree, making three motions. All vertebrates have the jaws opening vertically, while articulates have horizontal jaws, and the sea-urchin has converging jaws.



The harmony and mutual dependence between all parts of an animal's body are not better shown than in the adaptation of the teeth of vertebrates to the kind of food, and their consequent agreement with the habits, structure, and form of the individual. A single tooth frequently reveals the entire character of its possessor. Indeed, no single organ is of greater zoölogical value. Not only does the student of living forms use the teeth as indicative of character and a means of classification, but their superior durability makes them of the very greatest importance in the study of ancient life.

 

T is characteristic of architecture that the further back we look toward the primitive state of man, the more the use of wood prevails over that of stone, brick, and all other building materials, and the more does the importance of carpentry exceed that of masonry. The condition of this art might well be taken as the basis of division between the various stages of the civilization of a country. During the first stage, wood is everywhere the only material used in construction; the second stage is marked by a mixture of wood and ashlar work; while in the third, or highest stage, carpentry yields constantly to mason-work, until it becomes a mere subordinate agent. 