Mankind are all alike, merely exhibiting different degrees of culture. As the rills in the mountains born of the rains and the snows are all the same and reach the ocean by various devious and complicated courses, so the races of men, emerging from the darkness of the past, follow, because of the immutability of natural law, practically the same lines of development through savagery, barbarism, civilisation, toward a common goal of unification and enlightenment. The progress of humanity from earliest times to now appears to be divided, in each race evolution, into several epochs by certain great inventions or discoveries which seemed to spread themselves over the world either from one centre or from several. Of these the most important are, first, fire; second, the bow; third, smelting; fourth, phonetic writing; and fifth, printing. This progression is not even, but a people may stand still for a long time and then suddenly become active in one particular line, or in many lines.[37] Ours is the age of mechanical development; the Greeks made a stride in art. When development reaches a certain point and conditions are favourable for an invention, it springs into being not in one individual alone but usually in several widely separated ones, as if the seed of it had been sprinkled over the earth. It may have germinated before when conditions were not ripe, but it then died before even sprouting. Environment cultivates the mind, and the mind feeds on environment. Only a small portion of those to whom an idea occurs endeavour to carry it out, and often other subsequent inventions are necessary to success. [Illustration: PAINTED PETROGLYPHS, SANTA BARBARA COUNTY, CALIFORNIA] [Illustration: PETROGLYPHS IN BROWN’S CAVE, WISCONSIN] On the Amerind continent before the advent of the European the various stocks and tribes were rising and falling under the influence of the moulding conditions, and rising again or giving place to more highly vitalised stock which might succeed in fertilising in the brain of a Hiawatha or a Quetzalcohuatl great ideas that should lift them onward.