will tend to grow more decided.  If such transactions are from time to time repeated, these specialisations may become appreciable.  And whether or not there ensue distinct differentiations of other individuals into makers of particular articles, it is clear that incipient differentiations take place throughout the tribe:  the one original cause produces not only the first dual effect, but a number of secondary dual effects, like in kind, but minor in degree.  This process, of which traces may be seen among groups of schoolboys, cannot well produce any lasting effects in an unsettled tribe; but where there grows up a fixed and multiplying community, these differentiations become permanent, and increase with each generation.  A larger population, involving a greater demand for every commodity, intensifies the functional activity of each specialised person or class; and this renders the specialisation more definite where it already exists, and establishes it where it is nascent.  By increasing the pressure on the means of subsistence, a larger population again augments these results; seeing that each person is forced more and more to confine himself to that which he can do best, and by which he can gain most.  This industrial progress, by aiding future production, opens the way for a further growth of population, which reacts as before:  in all which the multiplication of effects is manifest.  Presently, under these same stimuli, new occupations arise.  Competing workers, ever aiming to produce improved articles, occasionally discover better processes or raw materials.  In weapons and cutting tools, the substitution of bronze for stone entails upon him who first makes it a great increase of demand—­so great an increase that he presently finds all his time occupied in making the bronze for the articles he sells, and is obliged to depute the fashioning of these to others:  and, eventually, the making of bronze, thus gradually differentiated from a pre-existing occupation, becomes an occupation by itself.

But now mark the ramified changes which follow this change.  Bronze soon replaces stone, not only in the articles it was first used for, but in many others—­in arms, tools, and utensils of various kinds; and so affects the manufacture of these things.  Further, it affects the processes which these utensils subserve, and the resulting products—­modifies buildings, carvings, dress, personal decorations.  Yet again, it sets going sundry manufactures which were before impossible, from lack of a material fit for the requisite tools.  And all these changes react on the people—­increase their manipulative skill, their intelligence, their comfort,—­refine their habits and tastes.  Thus the evolution of a homogeneous society into a heterogeneous one, is clearly consequent on the general principle, that many effects are produced by one cause.