Among them are some lactucas (or lettuces), the Chinese arbor vitæ, and a number of Australian plants. Whenever this peculiarity has been observed, it has also been found, on examination, that both sides of the leaves were structurally alike. The property can be brought out clearly under favorable conditions; but it is liable to be modified or marked, in the actual circumstances of growth, by any difference in exposure to the sun or wind, on different sides of the plant.
IT is almost one hundred years since the attention of T. R. Malthus was first called to the subject of population and its changes. As his views have had more influence than any other writer, it is well to notice briefly what they were. His leading principle is, that "population, when unchecked, increases in a geometrical ratio, while subsistence increases only in an arithmetical ratio." He held that "population is necessarily limited by the means of subsistence," and "invariably increases where those means increase, unless prevented by some very powerful and obvious check." He divides these checks into two classes, the positive and the preventive: among the former are wars, famine, diseases of all kinds, unhealthy occupations, extreme poverty, great cities, etc.; and in the latter class are abstinence from marriage and sexual intercourse, from considerations of prudence. The last class come more directly under the control of human agency.
The next writer of any note was Thomas Doubleday, who published in 1840 a work with this title: "The True Law of Population shown to be connected with the Food of the People." The term "true law" was undoubtedly introduced in opposition to the doctrine of Malthus. Doubleday attempted to demonstrate that "wherever a species or genus is endangered, a corresponding effort is invariably made by Nature for its preservation and continuance, by an increase of fecundity or fertility; and that this especially takes place whenever such danger arises from a diminution of proper nourishment," and that consequently "the deplethoric state is favorable to fertility." Thus, "there is in all societies a constant increase going on among that portion of it which is the class worst supplied with food—in short, among the poorest."
The April number of the "Westminster Review" for 1852 contained an elaborate essay by Herbert Spencer, introducing a "New Theory of Population," deduced from the general law of animal fertility. He "maintained that an antagonism exists between individualism and reproduction; that matter in its lower forms, for instance, of