Seed thrown on a soil naturally fertile but totally unprepared would be an
advance almost entirely lost. If it were once tilled the produce will be
greater; tilling it a second, a third time, might not merely double and triple,
but quadruple or decuple the produce, which will thus augment in a much larger proportion than the advances increase, and that up to a certain point, at which the produce will be as great as possible compared with the advances.
Past this point, if the advances be still increased, the produce will still
increase, but less, and always less and less until the fecundity of the earth
being exhausted and art unable to add anything further, an addition to the
advances will add nothing whatever to the produce.