Now, this reply shows that Mr Sadler does not in the least understand the principle which he has himself laid down. What is that principle? It is this, that the fecundity of human beings ON GIVEN SPACES, varies inversely as their numbers. We know what he means by inverse variation. But we must suppose that he uses the words, "given spaces," in the proper sense. Given spaces are equal spaces. Is there any reason to believe, that in those parts of Surrey which lie within the bills of mortality, there is any space equal in area to the space on which Guildford stands, which is more thickly peopled than the space on which Guildford stands? We do not know that there is any such. We are sure that there are not many. Why, therefore, on Mr Sadler's principle, should the people of Guildford be more prolific than the people who live within the bills of mortality? And, if the people of Guildford ought, as on Mr Sadler's principle they unquestionably ought, to stand as low in the scale of fecundity as the people of Southwark itself, it follows, most clearly, that they ought to stand far lower than the average obtained by taking all the people of Surrey together. The same remark applies to the case of Birmingham, and to all the other cases which Mr Sadler mentions. Towns of 5000 inhabitants may be, and often are, as thickly peopled "on a given space," as Birmingham. They are, in other words, as thickly peopled as a portion of Birmingham, equal to them in area. If so, on Mr Sadler's principle, they ought to be as low in the scale of fecundity as Birmingham. But they are not so. On the contrary, they stand higher than the average obtained by taking the fecundity of Birmingham in combination with the fecundity of the rural districts of Warwickshire.