to-day, part of the engine to-morrow, and a section of sleepers the day after. On our assumption then, the old railway is replaced by a new one after thirty years, a similar amount of labour being performed each year by the society so that simple reproduction takes place. But the railway can only be reproduced in this manner--it cannot be so produced. In order to make it fit for use and to make good its gradual wear and tear, the railway must have been completed in the first place. Though the railway can be repaired in parts, it cannot be made fit for use piecemeal, an axle to-day and a coach to-morrow. Indeed, the very essence of fixed capital is always to enter into the productive process in its entirety, as a material use-value. In order to get this use-form ready in the first place, society must apply a more concentrated amount of labour to its manufacture. In terms of our example, the labour of thirty years that is used for repairs, must be compressed into, say, two or three years. During this period of manufacture, society must therefore expend an amount of labour far greater than the average, that is to say it must have recourse to expanding reproduction; later, when the railway is finished, it may return to simple reproduction. Though we need not visualise the aggregate fixed capital as a single coherent use-object or a conglomeration of objects which must be produced all at once, the manufacture of all the more important means of production, such as buildings, transport facilities, and agricultural structures, requires a more concentrated application of labour, and this is true for the modern railway or steamship as much as it was for the rough stone-axe and the handmill. Therefore it is only in theory that simple reproduction can be conceived as alternating with enlarged reproduction; the latter is not only a general condition of a progressive civilisation and an expanding population, but also the _sine qua non_ for the economic form of fixed capital, or those means of production which in every society correspond to the fixed capital. Marx deals with this conflict between the formation of fixed capital and simple reproduction but indirectly, in connection with fluctuations in the wear and tear of the fixed capital, more rapid in some years than in others. Here he emphasises the need for perpetual 'over-production', i.e. enlarged reproduction, since a strict policy of simple reproduction would periodically lead to reproductive losses. In short, he regards enlarged reproduction under the aspect of an insurance fund for the fixed capital of the society, rather than in the light of the actual productive process.[87]