This is enough to show to what a sensitive young lady was exposed in illustrating "a principle as undeniable as the multiplication table," and in stating the facts upon which hangs the explanation of the poverty, and therefore of a large part of the vice and misery, of mankind. Miss Martineau's exposition was, of course, entirely right, and the fallacy in the review is obvious, one would suppose on the surface. The reviewer's error consists in his assumption--the falsity of which is at once apparent on the face of the statement--that land can go on doubling its produce _every_ ten years, for an indefinite period. So far from this being true, the fact is that the limit of improving the cultivation of land is soon reached. Better agricultural treatment may easily make half-cultivated land bring forth double its previous produce; but the highest pitch of farming once reached--as it comparatively soon is--the produce cannot be further increased; and even before this limit is reached, the return for each additional application of capital and labor becomes less and less proportionately bountiful. This is the truth known to political economists as "the Law of the Diminishing Return of Land." Taken in conjunction with the fact that the human race _can_ double for ever, theoretically, and in reality _does_ multiply its numbers with each generation, checked only by the forethought of the more prudent and the operations of famine, war, crime, and the diseases caused by poverty, this law explains why mankind does not more rapidly improve its condition--why the poor have been always with us--and why teaching such as Harriet Martineau here gave must be received into the popular mind before the condition of society can be expected to be improved in the only way possible, by the wisdom and prudence of its members. Painful as was the attack she had undergone, intensely as she had suffered from its character and nature, Miss Martineau did not allow what she had felt of personal distress to have any influence on her future writings. Her moral courage had been well trained and exercised, first by the efforts that her mind had had to make in following her conscience as a guide to the formation of opinions, in opposition to the tendency implanted by her mother's treatment to bow supinely before authority; secondly, by the lesson of endurance which her deafness had brought to her. She had now to show, for the first, but by no means the last time, that hers was one of those temperaments which belong to all leaders of men, whether in physical or moral warfare; that danger was to her a stimulus, and that her courage rose the higher the greater the demand for its exercise. Praise and blame, appreciation and defamation, strengthened and enlarged her mind during this period. But at the end of it, Sydney Smith could say: "She has gone through such a season as no girl before ever knew, and she has kept her own mind, her own manners, and her own voice. She's safe."