vegetables, possesses a stronger power of increase than in all higher forms; that the capacity of reproduction in animals is in an inverse ratio to their individuation; that the ability to maintain individual life and that of multiplication vary in the same manner also, and that this ability is measured by the development of the nervous system."
Fourier and some French writers have advanced the idea that "just in proportion as individuals become advanced in civilization, in the same proportion the race inclines to run out"; but whether this depends upon some change in physiological laws, or upon the influence of external agents, we are not informed. In establishing any law or general principle, it is highly important to understand distinctly what this principle is and its basis. During the present century, the above named persons are almost the only writers who have proposed anything like a general law or principle to guide the growth and changes of population.
The principle laid down by Herbert Spencer is the only one based strictly upon physiology. All the discussions and views of Malthus and Doubleday depend mainly upon food, climate, government, state of society, epidemics, war, etc. They make the leading factors, the primary agents in all these changes, outside, and in a great measure independent, of the body. It would seem more consistent with common sense, and all natural phenomena, that the law which governs the existence, growth, and changes of a living being should have its basis and development in that same organization. From observation and analogy, we believe such a doctrine exists throughout the whole animal and vegetable creation. The truth of this principle is strikingly illustrated in the changes that have taken place in domestic animals. The human system can not be made an exception to a universal principle.
This law of increase or propagation—the most important of all laws—must, in the very nature of things, be inherent in the body; must be incorporated into its very existence, though in its operations it may be affected by extraneous causes and influences. However powerful may be the effect of climate, food, and other external agents upon the application or working of this law, whether to impede, thwart, or modify its operation, the law must exist, we believe, in the body itself, and in a great measure control it. The various changes to which the human body is subjected, can not happen by chance or accident; neither can the causes be dissimilar or contradictory in different nations and races; neither can they radically change or vary from one generation to another. Universality and unchangeableness must characterize such a law. The reason why correct principles have not been brought to bear more directly upon the growth and changes of population is, that the principles of physiology were not formerly understood. The science was scarcely known at the time when