DARWIN AND THE ORIGIN OF SPECIES 237 



would assuredly take place in still waters, and we may fairly 

 believe that still waters then resembled still waters now. The 

 sufficiency of present actions is an excellent argument in the 

 absence of all proof of change, but falls to utter worthlessness 

 in presence of the direct evidence of change. We will try to 

 explain the nature of the evidence, which does prove not only 

 that the violence of all natural changes has decreased, but also 

 that it is decreasing, and must continue to decrease. 



Perpetual motion is popularly recognised as a delusion ; yet 

 perpetual 'motion is no mechanical absurdity, but in given con- 

 ditions is a mechanical necessity. Set a mass in motion and it 

 must continue to move for ever, unless stopped by something 

 else. This something else takes up the motion in some other 

 form, and continues it till the whole or part is again transmitted 

 to other matter ; in this sense perpetual motion is inevitable. 

 But this is not the popular meaning of '.perpetual motion,' which 

 represents a vague idea that a watch will not go unless it is 

 wound up. Put into more accurate form, it means that no finite 

 construction of physical materials can continue to do work for 

 an infinite time ; or in other words, one part of the construction 

 cannot continue to part with its energy and another part to 

 receive it for ever, nor can the action be perpetually reversed. 

 All motion we can produce in this world is accompanied by the 

 performance of a certain amount of work in the form of over- 

 coming friction, and this involves a redistribution of energy. 

 No continual motion can therefore be produced by any finite 

 chemical, mechanical, or other physical construction. In this 

 case, what is true on a small scale is equally true on a large 

 scale. Looking on the sun and planets as a certain complex 

 physical combination, differing in degree but not in kind from 

 those we can produce in the workshop by using similar materials 

 subject to the same laws, we at once admit that if there be no 

 resistance the planets may continue to revolve round the sun for 

 ever, and may have done so from infinite time. Under these 

 circumstances, neither the sun nor planets gain or lose a particle 

 of energy in the process. Perpetual motion is, therefore, in this 

 case quite conceivable. But when we find the sun raising huge 

 masses of water daily from the sea to the skies, lifting yearly 

 endless vegetation from the earth, setting breeze and hurricane 



