But if a surplus labouring population is a necessary product of accumulation
 or of the development of wealth on a capitalist basis, this surplus population
 becomes, conversely, the lever of capitalistic accumulation, nay, a condition
 of existence of the capitalist mode of production. It forms a disposable
 industrial reserve army, that belongs to capital quite as absolutely as if the
 latter had bred it at its own cost. Independently of the limits of the actual
 increase of population, it creates, for the changing needs of the
 self-expansion of capital, a mass of human material always ready for
 exploitation. With accumulation, and the development of the productiveness of
 labour that accompanies it, the power of sudden expansion of capital grows
 also; it grows, not merely because the elasticity of the capital already
 functioning increases, not merely because the absolute wealth of society
 expands, of which capital only forms an elastic part, not merely because
 credit, under every special stimulus, at once places an unusual part of this
 wealth at the disposal of production in the form of additional capital; it
 grows, also, because the technical conditions of the process of production
 themselves — machinery, means of transport, &c. — now admit of
 the rapidest transformation of masses of surplus-product into additional means
 of production. The mass of social wealth, overflowing with the advance of
 accumulation, and transformable into additional capital, thrusts itself
 frantically into old branches of production, whose market suddenly expands, or
 into newly formed branches, such as railways, &c., the need for which grows
 out of the development of the old ones. In all such cases, there must be the
 possibility of throwing great masses of men suddenly on the decisive points
 without injury to the scale of production in other spheres. Overpopulation
 supplies these masses. The course 593characteristic of modern industry,
 viz., a decennial cycle (interrupted by smaller oscillations), of
 periods of average activity, production at high pressure, crisis and
 stagnation, depends on the constant formation, the greater or less absorption,
 and the re-formation of the industrial reserve army or surplus population. In
 their turn, the varying phases of the industrial cycle recruit the surplus
 population, and become one of the most energetic agents of its reproduction.
 This peculiar course of modern industry, which occurs in no earlier period of
 human history, was also impossible in the childhood of capitalist production.
 The composition of capital changed but very slowly. With its accumulation,
 therefore, there kept pace, on the whole, a corresponding growth in the demand
 for labour. Slow as was the advance of accumulation compared with that of more
 modern times, it found a check in the natural limits of the exploitable
 labouring population, limits which could only be got rid of by forcible means
 to be mentioned later. The expansion by fits and starts of the scale of
 production is the preliminary to its equally sudden contraction; the latter
 again evokes the former, but the former is impossible without disposable human
 material, without an increase, in the number of labourers independently of the
 absolute growth of the population. This increase is effected by the simple
 process that constantly “sets free” a part of the labourers; by
 methods which lessen the number of labourers employed in proportion to the
 increased production. The whole form of the movement of modern industry
 depends, therefore, upon the constant transformation of a part of the labouring
 population into unemployed or half-employed hands. The superficiality of
 Political Economy shows itself in the fact that it looks upon the expansion and
 contraction of credit, which is a mere symptom of the periodic changes of the
 industrial cycle, as their cause. As the heavenly bodies, once thrown into a
 certain definite motion, always repeat this, so is it with social production as
 soon as it is once thrown into this movement of alternate expansion and
 contraction. Effects, in their turn, become causes, and the varying accidents
 of the whole process, which always reproduces its own conditions, take on the
 form of periodicity. When this periodicity is once consolidated, even Political
 Economy then sees that the production of a relative surplus population —
 i.e., surplus with regard to the average needs of the self-expansion
 of capital — is a necessary condition of modern industry.