80 INTRODUCTION TO EVOLUTION 



vitamins. But we note that living things are mainly composed of less than a 

 fifth of the elements available in nature, and that of this small group four 

 only are especially prominent. It may well be that this fundamental simi- 

 larity of chemical structure running through the plant and animal kingdoms 

 is basic to all other similarities. 



There is interest in the fact that the chemical elements found in living 

 things are among the most abundant of the elements found in nature. This 

 would be expected if living matter originated from nonliving matter. In 

 living things some elements, especially carbon and nitrogen, are present in 

 greater abundance than they are in the nonliving world, while other ele- 

 ments, e.g., silicon, are present in much less abundance. 



What do these chemical elements form in living organisms? The most 

 plentiful compound is wafer, composed of hydrogen and oxygen in the pro- 

 portion of two atoms of hydrogen to one of oxygen: H:.0. Water comprises 

 from 70 to 90 percent or more, by weight, of living things. It is a most es- 

 sential constituent, taking part in all life processes. Without it life could 

 not exist. Dissolved in the water are various inorganic salts, such as so- 

 dium chloride ("table salt"). 



In addition to water and inorganic salts, the chemical elements found in 

 largest amount in living things form three classes of organic compounds: 

 carbohydrates, fats, and proteins. Carbohydrates (e.g., starches, sugars, 

 and cellulose, the stiffening, fibrous material in plants) are composed of 

 carbon, hydrogen, and oxygen, the latter two elements being present in the 

 same proportions as they are in water. Fats are also composed of carbon, 

 hydrogen, and oxygen, but the oxygen constitutes a smaller proportion of 

 a fat molecule than it does of a carbohydrate molecule. Other elements, 

 such as nitrogen and phosphorus, may also be present in fats. Proteins, fa- 

 miliar to us in "lean meat" (muscle), are the most complex substances 

 known. They always contain carbon, hydrogen, oxygen, nitrogen, and sul- 

 fur. Phosphorus is also a frequent constituent, having a special duty to per- 

 form, as we shall see shortly. Other elements may be present. 



Again we note the fundamental similarity of all living things, expressed 

 in this case by the fact that they are all composed of proteins, carbohy- 

 drates, and fats, which combine to form the complex substance termed by 

 Huxley "the physical basis of life": protoplasm. Protoplasm, "the living 

 stuff," is observable with the microscope and is found to have many of the 

 same physical and chemical properties in all living things. Actually, there 

 are differences. The protoplasms of plant, amoeba, and man are not identi- 

 cal. But the similarities overshadow the differences, indicating again that 

 all life is one. 



