CONNECTION OF CIIKMICAL AND VITAL ACTIVITY. 225 



chemical and tlioso which we term vital, and apparently in 

 much the same degree ; but wliether they modify tlie vital only 

 through the chemical, we are at present unable to say. In- 

 stances mny readily be found in which life continues active, 

 although one or other of the forces mentioned is not supplied to 

 the living body from without, and is only secondarily present as 

 a result of chemical changes going on within. Thus a seed in 

 the earth, a fungus in a cellar, or a proteus in its dark cave, 

 live and thrive without a ray of light ; and the whale and w^al- 

 rus in the Arctic seas are independent of any external heat, 

 their temperature being only maintained by combustion taking 

 place within their own bodies. But there seems to be no in- 

 stance of vitality alone continuing active when a stop has been 

 put to the occurrence of chemical changes. Sometimes both 

 chemical and vital processes are suspended together for a time, 

 as in a grain of wheat or a rotifer when it is kept diy, or in an 

 egg when kept cool and coated with varnish to exclude air. So 

 long as chemical activity remains dormant, no other form of 

 energy can awake the latent vitality. Only when the conditions 

 necessary for chemical transformation of the proper kind and 

 amount are supplied, does it again become manifest. Thus 

 light or heat may be applied in any amount or any proportion 

 to an egg, a seed, or a dried rotifer, and still they will not grow 

 or move if the air be withheld from the former or the moisture 

 from the two latter, which is essential for the production of 

 chemical changes within them. 



Conditions of Health and Disease. — These changes of which 

 I have been speaking consist in the assimilation of certain sub- 

 stances, their decomposition within the organism, and the re- 

 jection of waste products. A due proportion between these 

 constitutes health. Just as a fire can only be kept bright by 

 raking out the ashes and supplying fresh fuel as that in the 

 ^rate burns away, so an organism can only be kept healthy 

 by removing the products of waste and supplying fresh nutri- 

 ment as its tissues get decomposed during action. The condi- 

 tions necessary for this purpose are secured in the simplest 

 forms of life, such as the amoeba, by the little mass of pro- 

 toplasm moving about in a fluid, which can not only supply 



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