CHAPTER V. 



THE CORRESPONDENCE BETWEEN LIFE AND ITS 

 CIRCUMSTANCES. 



§ 27. We habitually distinguish between a live object 

 and a dead one, by observing whether a change which we 

 make in the surrounding conditions, or one which Nature 

 makes in them, is or is not followed by some perceptible 

 change in the object. By discovering that certain things 

 shrink when touched, or fly away when approached, or start 

 when a noise is made, the child first roughly discriminates 

 between the living and the not-living; and the man when in 

 doubt whether an animal he is looking at is dead or not, stirs 

 it with his stick; or if it be at a distance, shouts, or throws a 

 stone at it. Vegetal and animal life are alike primarily 

 recognized by this process. The tree that puts out leaves 

 when the spring brings increase of temperature, the flower 

 which opens and closes with the rising and setting of the 

 sun, the plant that droops when the soil is dry and re-erects 

 itself when watered, are considered alive because of these in- 

 duced changes; in common with the acorn-shell which con- 

 tracts when a shadow suddenly falls on it, the worm that 

 comes to the surface when the ground is continuously shakien, 

 and the hedgehog that rolls itself up when attacked. 



. Xot only, however, do we look for some response when an 

 external stimulus is applied to a living organism, but we 

 expect a fitness in the response. Dead as well as living 

 things display changes under certain changes of condition; 

 instance, a lump of carbonate of soda that effervesces when 

 dropped into sulphuric acid; a cord that contracts when 

 wetted; a piece of bread that turns brown when held near 



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