DARWIN AND THE ORIGIN OF SPECIES 239 



same manner as the complete cycle of any mechanical operations 

 could be followed. All the pressures and resistances of the 

 machine were calculable; we knew not so much as if there were 

 analogous pressures and resistances in nature's mechanism. 

 The establishment of the doctrine of conservation of energy, 

 showing a numerical equivalence between the various forms of 

 physical energy exhibited by vis viva, heat, chemical affinity, 

 electricity, light, elasticity, and gravitation, has enabled us to 

 examine the complete series of any given actions in nature, 

 even as the successive actions of a train of wheels in a mill can 

 be studied. There is no missing link ; there is no unseen 

 gearing, by which, in our ignorance, we might assume that the 

 last wheel of the set somehow managed to drive the first. We 

 have experimentally proved one law that the total quantity of 

 energy in the universe is constant, meaning by energy some- 

 thing perfectly intelligible and mensurable, equivalent in all 

 cases to the product of a mass into the square of a velocity, some- 

 times latent, that is to say, producing or undergoing no change ; 

 at other times in action, that is to say, in the act of producing or 

 undergoing change, not a change in amount, but a change of 

 distribution. First, the hand about to throw a ball, next the 

 ball in motion, lastly the heated wall struck by the ball, con- 

 tain the greater part of the energy of the construction ; but, 

 from first to last, the sum of the energies contained by the hand, 

 the ball, and the wall, is constant. At first sight, this constancy, 

 in virtue of which no energy is ever lost, but simply transferred 

 from mass to mass, might seem to favour the notion of a possible 

 eternity of change, in which the earlier and later states of the 

 universe would differ in no essential feature. It is to Professor 

 Sir W. Thomson of Glasgow that we owe the demonstration of 

 the fallacy of this conception, and the establishment of the 

 contrary doctrine of a continual dissipation of energy, by which 

 the available power to produce change in any finite quantity of 

 matter diminishes at every change of the distribution of energy. 

 A simple illustration of the meaning of this doctrine is afforded 

 by an unequally heated bar of iron. Let one end be hot and 

 the other cold. The total quantity of heat (representing one 

 form of energy) contained by the bar is mensurable and finite, 

 and the bar contains within itself the elements of change the 



