We do not yet possess, probably we shall never possess, any means of estimating even within five or six thousand years, the actual duration of the stone age. From all analogies progress must have been, in the beginning, exceedingly slow; like that of a falling body, the rapidity of industrial progress is continually accelerating. This acceleration is not of course quite regular; the phenomena of social life are too complex, the forces at work are too numerous and sometimes too contrary to allow us to express it by the mathematical formula which may be applied to movement in the physical world; but on the whole this law of constantly accelerated progress holds good, as indeed may be historically proved. So long as man had to do without metals, each generation, in all probability, added but little to the discoveries of that which preceded it; most likely after each happy effort many generations succeeded one another without any further attempt to advance. Ever since they have been under our observation, the savage races of the world have been practically stationary except where European commerce has profoundly modified the conditions of their lives. It is probable, therefore, that more centuries rolled away between the first chipped flints and the well polished weapons which succeeded them than between the latter and the earliest use of bronze. But we cannot prove that it was so, nor satisfy those whom probability and a specious hypothesis will not content. Where neither written evidence nor oral tradition exist there can be little question of historic order. The remains of the stone age are not calculated to dissipate the silence which enshrouds those centuries. In the art of a civilized people we find their successive modes of feeling and thought interpreted by expressive forms; we may even attempt under all reserve to sketch their history with the sole aid of their plastic remains. The chances of error would of course be numerous; but yet if all other materials had, unhappily, failed us, the attempt would have been well worth making. The more ancient portions of our prehistoric collections do not offer the same opportunities; they are too simple and too little varied. The primitive savage who moulded matter to his will with great and painful difficulty, could impress upon it nothing but those gross instincts which are common to man and beast; we can discover nothing from his works, beyond the means which he employed in his struggles with his enemies, and in his never-ending effort to procure food for himself.