NATURE AND OEIGIN OF LIFE. 17 



of the mechanism stimulated, and on the amount of 

 energy stored in it. But the mechanism may be so dis- 

 posed as to vary in its response according to the intensity 

 and duration of the stimulus. An external stimulus fires 

 off, so to speak, an internal metabolic change ; this may 

 give rise to other changes, some of which may react on 

 the first. So the motion of a mechanism may regulate 

 its own action ; as does, for instance, the governor of a 

 steam-engine. An internal change may become an in- 

 ternal stimulus starting new reactions. But there is no 

 real distinction between internal and external stimuli ; 

 - 4 vhat is external to a part may be internal to the whole. 



We must think, then, of living organisms as marvel- 

 lously complex mechanisms, with their parts so adjusted 

 as to set going, regulate, or restrict each other's action. 

 A chain of such interactions forms the self-repairing, 

 self-regulating mechanism so essential for the continu- 

 ance of the process of life. 



It is by means of this fundamental attribute of irrita- 

 bility that protoplasm comes into relation with its en- 

 vironment. It is the secret of adaptation. Of the 

 stimulating factors of the environment some, like tem- 

 perature and water, are of a general nature, ever present 

 and necessary ; others, like sound or light or some par- 

 ticular chemical compound, are more special, and not 

 always essential. 



When first we approach the problems of evolution we 

 are apt to ask for definitions ; to seek for distinctions 

 separating the living from the dead, the organic from the 

 inorganic : we try to discover hard and fast lines between 

 species and varieties, between plants and animals, be- 

 tween the conscious and the unconscious. But as we 

 study the question deeper, and extend our field of vision, 

 we come to recognise that the definitions are usually 

 misleading, the distinctions artificial, the sharp lines 

 arbitrary. The breaks in nature, if breaks there seem 

 to be, are gaps in our knowledge, and diminish in size and 



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