Then, having acquired the habit of thinking of a building as having originated in a plan, we will follow the building as it grows up out of the plan, taking vertical form in what the architect calls the _elevation_, or, when he is speaking specifically of the outside of the building, the _façades_. Sometimes we shall study one of the diagrams, which he calls a _section_, when he imagines his building intersected by a vertical plane that cuts the structure into two parts. The one between the spectator and the cutting plane is supposed to be removed, and thus is laid bare the system of the interior construction-work. In studying the exterior of a building, therefore, we shall keep in mind the interior disposition, arising out of the planning, and acquire the habit of looking on the outside of a building as logically related to the interior. The design of a building will come to mean to us not a mere pattern of façade, arbitrarily invented, but an arrangement of vertical and horizontal features, of solid surfaces and open spaces, that has grown out of the interior conditions and proclaims them. In a word, we shall regard a work of architecture as an organic growth; rooted in the plan, springing up in accordance with constructive principles; each part having its separate function, and all co-ordinated in harmonious relation to the unity of the whole. For we shall find that _unity of design_ is a special element of excellence in architecture; a unity secured by the relations of _proportion, harmony and rhythm_ established between the several parts and between the parts and the whole. And, since architecture is primarily an art of practical utility, all these relations are equally determined by the principle of _fitness_; in order that each and every part may perform most efficiently its respective function in the combined purpose of the whole edifice. For this is the first and final criterion of organic composition. [Illustration: STONEHENGE. SALISBURY PLAIN, ENGLAND PRIMITIVE USE OF POST AND BEAM CONSTRUCTION. PP. 8, 16] [Illustration: SECTION AND PLAN OF “TREASURY OF ATREUS” AT MYCENÆ. EXAMPLE OF “THOLOS” OR BEE-HIVE CONSTRUCTION. P. 14] [Illustration: TEOCALLI OR “HOUSE OF GOD” AT GUATUSCO, COSTA RICA. P. 20] CHAPTER II PRIMITIVE STRUCTURES The various remains that exist of prehistoric structures, though scattered widely over different parts of the world, present a general similarity of purpose and design.