74 APHORISMS AND REFLECTIONS 



which have grown out of this. But coal is as much 

 an essential condition of this growth and development 

 as carbonic acid is for that of a club-moss. Want- 

 ing coal, we could not have smelted the iron needed 

 to make our engines, nor have worked our engines 

 when we had got them. But take away the 

 engines, and the great towns of Yorkshire and 

 Lancashire vanish like a dream. Manufactures give 

 place to agriculture and pasture, and not ten men 

 can live where now ten thousand are amply sup- 

 ported. 



Thus, all this abundant wealth of money and of 

 vivid life is Nature's interest upon her investment in 

 club-mosses, and the like, so long ago. But what 

 becomes of the coal which is burnt in yielding this 

 interest ? Heat comes out of it, light comes out of 

 it ; and if we could gather together all that goes up 

 the chimney, and all that remains in the grate of a 

 thoroughly-burnt coal-fire, we should find ourselves 

 in possession of a quantity of carbonic acid, water, 

 ammonia, and mineral matters, exactly equal in 

 weight to the coal. But these are the very matters 

 with which Nature supplied the club-mosses which 

 made the coal. She is paid back principal and 

 interest at the same time ; and she straightway 

 invests the carbonic acid, the water, and the ammonia 

 in new forms of life, feeding vvith them the plants 

 that now live. Thrifty Nature ! Surely no prodigal, 

 but most notable of housekeepers ! 



CCVII 



Here, then, is a capital fact. The movements of 

 the lobster are due to muscular contractility. But 

 ■why does a muscle contract at one time and not at 

 another? Why does one whole group of muscles 

 contract when the lobster wishes to extend his tail, 

 and another group when he desires to bend it ? 

 What is it originates, directs, and controls the motive 

 power ? 



Experiment, the great instrument for the ascer- 



