246 SPH^EKELLA CHAP. 



has shown that this oxygen is produced by the decom- 

 position of the carbon dioxide contained in solution in 

 rain-water, and indeed in all water exposed to the air ; 

 the gas, which is always present in small quantities in 

 the atmosphere, being very soluble in water. 



As the carbon dioxide is decomposed in this way, its 

 oxygen being given off, it is evident that its carbon must 

 be retained. As a matter of fact it is retained by the 

 organism but not in the form of carbon ; in all probability 

 a double decomposition takes place between the carbon 

 dioxide absorbed and the water contained in its proto- 

 plasm, the result being the liberation of oxygen in the 

 form of gas and the simultaneous production of some 

 extremely simple form of carbohydrate, i.e., some com- 

 pound of carbon, hydrogen, and oxygen with a com- 

 paratively small number of atoms to the molecule. 



The next step seems to be that the carbohydrate thus 

 formed unites with the ammonia salts or the nitrates 

 absorbed from the surrounding water, the result being 

 the formation of some comparatively simple nitrogenous 

 compound. Then further combinations take place, 

 substances of greater and greater complexity are pro- 

 duced, sulphur from the absorbed sulphates enters into 

 combination, and proteids are formed. From these, 

 finally, fresh living protoplasm arises. 



From the foregoing account, which only aims at giving 

 the very briefest outline of a subject as yet imperfectly \ 

 understood, it will be seen that, as in Amoeba, the final i 

 result of the nutritive process is the manufacture of 

 protoplasm, and that this result is attained by the form- 

 ation of various substances of increasing complexity. 

 But it must be noted that the steps in this process of 

 constructive metabolism are widely different in the two 

 cases. In Amoeba we start with living protoplasm 

 that of the prey which is killed and broken up into 



