The Conditions of Life and Death. 83 



by the growing disbelief in spontaneous generation, 

 which dates from Redi's experiments (1626-97). 



(3.) The third period, which practically dates from 

 the establishment of the doctrine of the conservation of 

 energy, is marked by the realization of the organism's 

 complete dependence upon its environment, and by the 

 disappearance of the doctrine of a special vital force. 

 But although a conviction has grown that the living 

 and the not-living differ in degree rather than in kind, 

 it is confessed by those who are frank that the secret 

 of the synthesis which is expressed in living matter or 

 in the organism remains undiscovered. 



To the careless, it may seem that nothing could be 

 easier than to distinguish the living organism from its 

 non-animate physical surroundings. But is 

 there anything more difficult ? (^ and & th"e 



Are we inclined to lay emphasis on form ^oifd" 10 

 and structure? Then we recall the exquisite 

 beauty of some dendritic minerals and the formlessness 

 of the amoeba, the complexity of many crystals and the 

 apparent simplicity of a slime fungus. 



Or is it the power of growth that impresses us as 

 characteristic of the organism? What then of the in- 

 organic crystal which grows beautifully under our eyes? 

 Or if it is development, the passing from stage to stage, 

 that characterizes life, what then of the vapour that 

 passes into the form of a snow-flake which is dissolved 

 again into water. 



Is the organism a material system, with the power of 

 changing matter and energy from one form to another 

 and doing work thereby? But so, in truth, is the 

 steam-engine. 



Is it the power of movement that we would emphasize 

 as characteristic of life? What then of the fragment of 

 potassium darting hither and thither on the surface of 

 the water? 



Is it irritability that characterizes life? But what is 

 irritability but the power of responding to stimulus, and 

 surely even the barrel of gunpowder will do that? 



Nor is any chemical characteristic of the living organ- 

 ism at first sight apparent. It is certain that there is 



