Thirdly: A farmer may invest additional capital in the same leasehold even if the additional product secured in this manner yields him only the price of production at the prevailing market-prices, i.e., provides him with the usual profit but does not enable him to pay any additional rent. He thus pays ground-rent with one portion of the capital invested in the land, but not with the other. How little this assumption helps to solve the problem, however, is seen from the following: If the market-price (and the fertility of the soil) enables him to obtain an additional yield with his additional capital, which, as in the case of the old capital, yields a surplus-profit in addition to the price of production, he is able to pocket this surplus-profit so long as his lease does not expire. But why? Because the limitation placed by landed property on the investment of his capital in land has been eliminated for the duration of the lease. But the simple fact that additional soil of poorer quality must be independently cleared and independently leased in order for him to secure this surplus-profit proves irrefutably that the investment of additional capital in the old soil no longer suffices to produce the required increased supply. One assumption excludes the other. It is true that now one might say: The rent on the worst soil A is itself differential rent — whether the comparison is made with respect to the land cultivated by the owner himself (this occurs, however, as a purely chance exception) or with respect to the additional investment of capital in the old leaseholds which do not yield any rent. However, this would be 1) a differential rent which does not arise from the difference in fertility of the various categories of soil, and which therefore would not presuppose that soil A does not yield any rent and its produce sells at the price of production; and 2) the circumstance whether additional investments of capital in the same leasehold yield rent or not is just as irrelevant to the question as to whether the new soil of class A to be taken under cultivation pays rent or not, as it is irrelevant to, say, the establishment of a new and independent manufacturing business whether another manufacturer in the same line invests a portion of his capital in interest-bearing paper because he cannot use all of it in his business, or whether he makes certain improvements which do not yield him the full profit, but nevertheless do yield more than interest. This is of secondary importance to him. The additional new establishments, on the other hand, must yield the average profit and are organised in the hope of obtaining this average profit. It is true, to be sure, that the additional investments of capital in the old leaseholds and the additional cultivation of new land of soil type A mutually restrict one another. The limit, up to which additional capital may be invested in the same leasehold under less favourable conditions of production, is determined by the competing new investments in soil A; on the other hand, the rent which this category of soil can yield is limited by the competing additional investments of capital in the old leaseholds.