4 o THE POPULAR SCIENCE MONTHLY. 



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 sys- 

 tem." 



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 ex- 

 ternal agents, we are not informed. In establishing any law or gen- 

 eral 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 any- 

 thing 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 com- 

 mon 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 obser- 

 vation 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 do- 

 mestic 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 



