7 22 THE POPULAR SCIENCE MONTHLY. 



producing equilibrium and stability ; tbe otber concerns the progressive 

 movement of the system, as a whole, to higher and higher conditions 

 the movement of the point of equilibrium itself, by constant slight dis- 

 turbance and readjustment of parts on a higher plane, with more com- 

 plex inter-relations. The one concerns the laws of sustentation of the 

 system, the other the laws of evolution. The one concerns things as 

 they are, the other the process by which they become so. Now, Nature 

 as a whole is such a system of correlated parts. Every department and 

 sub-department of Nature, whether it be the solar system or the earth, 

 or the organic kingdom, or human society, or the human body, is such 

 a system of correlated parts, and is therefore subject to evolution. We 

 can best make this thought clear by examples : 



1. Take, then, the human body. This complex and beautiful sys- 

 tem of correlated and nicely-adjusted parts may be studied in a state 

 of maturity and equilibrium, in which all the organs and functions by 

 action and reaction co-operate to produce perfect stability, health, and 

 physical happiness. This study is physiology. Or else the same may 

 be studied in a state of progressive change. Now, we perceive that 

 the stability is never perfect the point of equilibrium is ever moving. 

 By the ever-changing number and relative power of the co-operating 

 parts the equilibrium is ever being disturbed, only to be readjusted on 

 a higher plane, with still more beautiful and complex inter-relations. 

 This is growth, development, evolution. Its study is called embry- 

 ology. 2. Take another example the solar system. "We may study 

 sun, planets, and satellites in their mutual actions and reactions, co- 

 operating to produce perfect equilibrium, stability, beautiful order, 

 and musical harmony. This is the ideal of physical astronomy as em- 

 bodied in Laplace's " Mecanique Celeste." Or we may study the same 

 in its origin and progressive change. Now, we perceive that equilib- 

 rium and stability are never absolutely perfect, but, on the contrary, 

 there is continual disturbance with readjustment on a higher plane 

 continual introduction of infinitesimal discord, only to enhance the 

 grandeur and complexity of the harmonic relations. This is the nebu- 

 lar hypothesis the theory of the development of the solar system. It 

 is cosmogony ; it is evolution. 3. Again : society may be studied in 

 the mutual play of all its social functions so adjusted as to produce 

 social equilibrium, happiness, prosperity, and good government. This 

 is social statics. But equilibrium and stability are never perfect. 

 Permanent social equilibrium would be social stagnation and decay. 

 Therefore, we must study society also in its onward movement the 

 equilibrium ever disturbed, only to be readjusted on a higher plane 

 with more and more complexly inter-related parts. This is dynamics 

 social progress. It is evolution. 4. Again : the earth, as a whole, 

 may be studied in its present forms, and the mutual action of all its 

 parts lands and seas^ mountains and valleys, rivers, gulfs, and bays, 

 currents of air and ocean and the manner in which all these, by action 



