It is sometimes thought when simple contrivances such as weapons of stone and bronze, some of the simpler kinds of ornaments, and of tools obviously adapted to primeval life are found to extend over wide areas, and in places very remote from one another, that the few ideas necessary for the construction and use of them might easily have suggested themselves independently in different places. To the student of primitive culture who has become impressed with the persistency of art forms, this independent origin of such things does not appear so certain even in the case of the most simple contrivances. But when we come to a complex piece of mechanism, such as a spring padlock having several parts--the spring, the case, the parallel bar, and the key, in all of which the resemblance is maintained in distant countries, and which, with slight modification and continuously progressive improvements, are put together in the same manner in all parts of the world--such a supposition cannot be admitted, the necessity for a common origin is apparent, and the study of the periods and the circumstances connected with the distribution of it cannot be set aside as superfluous. Assuming that the tumbler pin-lock and the spring padlock cannot be traced back earlier in Europe than the commencement of our era, it is by no means certain that they may not have existed earlier elsewhere. The commerce carried on with the East in early times was of a nature to render it very probable that any contrivance for securing goods should have spread from place to place with the merchandise exported and imported between China, India, and Europe. A brief survey of the trade relations between different countries will be sufficient to show this.