30 GENERAL ZOOLOGY 



phosphates, chlorides, and carbonates of calcium, potas- 

 sium, magnesium, and iron. Some jelly fishes are more than 

 99 per cent, water. 



Organic compounds were formerly believed to come only 

 from living things but now the chemist may make thou- 

 sands of them in his laboratory, and many are of vast 

 commercial importance. The chief classes of organic com- 

 pounds are proteins, carbohydrates, and fats. Proteins 

 differ from the others in that they contain nitrogen in 

 addition to carbon, oxygen, and hydrogen. They consti- 

 tute the greater, and most essential, part of the protoplasm. 

 Lean meat and white of egg are examples of substances 

 which are largely made up of proteins. Carbohydrates, of 

 which sugars and starches are examples, have their oxygen 

 and hydrogen in the same proportions (H 2 O) in which they 

 occur in water. Fats, or hydrocarbons, contain the same 

 elements as carbohydrates but in different proportions, so 

 that there is a smaller percentage of oxygen. 



It is possible for the chemist to analyze protoplasm to 

 some extent and to learn the exact formula? for some of the 

 chemical compounds present, but its protein molecules are 

 so large that their exact composition and arrnagement is 

 still a matter of uncertainty in most cases. Here again, 

 then, we are met with an unsatisfactory termination in our 

 search for the fundaments of life. When we try to analyze 

 living substance, it dies, and we have left a number of com- 

 plex compounds (Fig. 19) which, though perhaps simpler 

 than those present during life, are still of such complicated 

 structure that they are imperfectly known. 



MECHANISM AND VITALISM 



Notwithstanding our comparative ignorance concerning 

 the phenomena of life men have not been backward in 

 putting forth theories to explain the operation of living 

 things. There has been long and bitter controversy be- 

 tween the mechanists, who maintain that living matter is 



