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CHAPTER II. 

 PHENOMENA AND CONDITIONS OF LIFE. 



THE Ancients regarded motion as the most important 

 criterion of life. ' All that moves lives.' The murmuring well, 

 the burning fire, the rapid lightning were to them living beings. 

 The same conception governs the untaught even to-day. We 

 read in ' Eobinson Crusoe ' that Friday, the child of Nature, 

 who has never come in contact with civilization and does not 

 know the use of fire, explains to himself the bubbling of the 

 boiling water by believing that it is stirred up by an animal 

 which is hiding in the kettle. 



Motion is doubtless the most prominent characteristic of 

 living substance, and we may define life as a series of extremely 

 complex and varied motions. But for all that the conception of 

 motion is not identical with the conception of life nor are we 

 able to say of any form of motion that it is specific to life, 

 for as there are inorganic bodies the parts of which are in active 

 motion, so there are living organisms which exhibit no trace of 

 any perceptible movement. It often needs prolonged investiga- 

 tion to decide whether a certain object is dead or alive ; super- 

 ficial inspection frequently leads to wrong conclusions. 



If we observe under the microscope a drop of human blood 

 we perceive at first in the fluid only the red and white corpuscles. 

 But soon numerous minute, round, pear- or star-shaped par- 

 ticles become visible, which dart about with a constant vibrat- 

 ing motion. No one would doubt that these minute motile 

 bodies are alive, for as no external cause of their movements is 

 perceptible we seem justified in assuming that they are actively 

 motile. But this assumption would be wrong, for these minute 

 bodies are nothing but the products of decomposition of the red 

 blood corpuscles, and their motion is due to the molecular 



