218 SCIENTIFIC THOUGHT. 



of modern physics regarding the phenomena of animal 

 movements, gradually substituted for the miracles of 

 the ' vital forces ' a molecular mechanism, complicated, 

 indeed, and likely to baffle our efforts for a long time to 

 come, but intelligible, nevertheless, as a mechanism. The 

 third achievement to which I refer is the revival among 

 us by Helmholtz and Mayer of the doctrine of the con- 

 servation of force. This cleared up the conception of 

 force in general, and in particular supplied the key to a 

 knowledge of the change of matter in plants and animals. 

 By this an insight was gained into the truth that the 

 power with which we move our own limbs (as George 

 Stephenson did those of his locomotive) is nothing more 

 than sunlight transformed in the organism of the plant : 

 that the highly oxygenated excrements of the animal 

 organism produce this force during their combustion, and 

 along with it the animal warmth, the irvev/uia of the 

 39. ancients. In the daylight which through such know- 

 force "aban- ledge penetrated into the chemical mechanism of plants 



doned. 



and animals, the pale spectre of a vital force could no 

 more be seen. Liebig, indeed, who himself stood up so 

 firmly for the chemical origin of animal heat and motive 

 power, still retains an accompanying vital force. But 

 this contradiction is probably to be traced to the cir- 

 cumstance that the celebrated chemist came late, and as 

 it were from outside, to the study of the phenomena of 

 life. And even Wohler still believes in a vital force, he 

 who in his time did more than any one to disturb the 

 vitalistic hypothesis through his artificial production of 

 urea." 



