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Nomadic tribes do not permanently expose any groups of their members to speciallocal conditions; nor does a stationary tribe, when occupying only a smallarea, maintain from generation to generation marked contrasts in the localconditions of its members; and in such tribes there are no decided economicdifferentiations. But a community which, by conquest, or otherwise, has overspreada large tract, and has become so far settled that its members live and diein their respective districts, keeps its several sections in different circumstances;and then they no longer remain alike in their occupations. Those who livedispersed continue to hunt or cultivate the earth; those who spread to thesea-shore fall into maritime occupations; while the inhabitants of some spotchosen, perhaps for its centrality, as one of periodic assemblage, becometraders, and a town springs up. In the adaptations of these social unitsto their respective functions, we see a progress from uniformity to multiformitycaused by unlike incidence of forces. Later in the process of social evolutionthese local adaptations are greatly multiplied. Differences in soil and climate,cause the rural inhabitants in different parts of the kingdom to have theiroccupations partially specialized, and to be come known as chiefly producingcattle, or sheep, or wheat, or oats, or hops, or fruit. People living wherecoal-fields are discovered are transformed into colliers; Cornishmen taketo mining because Cornwall is metalliferous; and iron-manufacture is thedominant industry where iron-stone is plentiful. Liverpool has taken to importingcotton, because of its proximity to the district where cotton-goods are made;and for analogous reasons Hull has become the chief port at which foreignwools are brought in. Thus in general and in detail, industrial heterogeneitiesof the social organism primary depend on local influences. Those divisionsof labour which, under another aspect, were interpreted as due to the settingup of motion in the directions of least resistance (§80), are here interpretedas due to differences in the incident forces; and the two interpretationsare quite consistent with each other. For that which in each determines thedirection of least resistance, is the distribution of the forces to be overcome;and hence unlikenesses of distribution in separate localities, entails unlikenessesin the lines of human actions in those localities -- entails industrial differentiations. §155. It has still to be shown that this general truth is demonstrablea priori -- that the instability of the homogeneous is a corollary from thepersistence of force. Already this has been tacitly implied, but here itwill be proper to expand the tacit implication into definite proof.

On striking a mass of matter with such force as either to indent it ormake it fly to pieces, we see both that the blow affects differently itsdifferent parts, and that the differences are consequent on the unlike relationsof its parts to the force impressed. The part struck is driven in towardsthe centre of the mass. It thus compresses, and tends to displace, the morecentrally situated portions. These, however, cannot be compressed or thrustout of their places without pressing on surrounding portions. And when theblow is violent enough to fracture the mass, we see, in the radial dispersionof the fragments, that the original momentum has been divided into numerousminor momenta, unlike in their directions. We see that the parts are differentlyaffected by the disruptive force, because they are differently related toit in their directions and attachments -- that the effects being the jointproducts of the force and the conditions cannot be alike in parts which aredifferently conditioned. A body on which radiant heat is falling, exemplifiesthis truth still more clearly. Take the simplest case -- that of a sphere.

While the part nearest to the radiating centre receives the rays at rightangles, the rays strike the other parts of the exposed side at all anglesfrom 90° down to 0°. The molecular vibrations propagated throughthe mass from the surface which receives the heat, proceed inwards at anglesdiffering for each point. Further, the interior parts reached by the vibrationsproceeding from all points of the heated side, must be dissimilarly affectedin proportion as their positions are dissimilar. So that whether they beon the recipient area, in the middle, or at the remote side, the constituentmolecules are thrown into states of vibration more or less unlike one another.