Or, on the other hand, the rent rises because the mass of capital exploiting
 the land increases, assuming that the relative productivity and market-price
 remain the same. Although the rent thus remains the same compared with the
 invested capital, still its mass, for instance, may be doubled, because the
 capital itself has doubled. Since no fall in price has occurred, the second
 investment of capital yields a surplus-profit just as well as the first, and it
 likewise is transformed into rent after the expiration of the lease. The mass
 of rent rises here, because the mass of capital producing a rent increases. The
 contention that various successive investments of capital in the same piece of
 land can produce rent only in so far as their yield is unequal, so that a
 differential rent thus arises, is reduced to the contention that when two
 capitals of £1,000 each are invested in two fields of equal productivity, only
 one of them can produce a rent, although both fields belong to a better soil
 type, which produces differential rent. (The mass of rental, the total rent of
 a country, grows therefore with the mass of capital invested, without the price
 of the individual pieces of land, or the rate of rent, or even the mass of rent
 on individual pieces of land, necessarily increasing; the amount of rental
 grows in this case with the extension of cultivation over a wider area. This
 may even be combined with a decrease in rent on individual holdings.)
 Otherwise, this contention would lead to the other, namely, that the investment
 of capital in two different pieces of land existing side by side follows
 different laws than the successive investment of capital in the same plot,
 whereas differential rent is derived precisely from the identity of the law in
 both cases, from the increased productiveness of capital invested either in the
 same field or in different fields. The only modification which exists here and
 is overlooked is that successive investments of capital, when applied to
 different pieces of land, meet the barrier of landed property, which is not the
 case with successive investments of capital in the same piece of land. This
 accounts for the opposing tendencies by which these two different forms of
 investment curb each other in practice. No difference in capital ever appears
 here. If the composition of the capital remains the same, and similarly the
 rate of surplus-value, the rate of profit remains unaltered, so that the mass
 of profit is doubled when the capital is doubled. In like manner the rate of
 rent remains the same under the assumed conditions. If a capital of £1,000
 produces a rent of x, then a capital of £2,000, under the assumed conditions,
 produces a rent of 2x. But calculated with reference to the area of land, which
 has remained unaltered, since, according to our assumption, the doubled capital
 operates in the same field, the level of rent has also risen as a consequence
 of its increase in mass. The same acre which yielded a rent of £2, now yields
 £4. [41]