84 INTRODUCTION TO EVOLUTION 



the organism in which they are found. Perhaps the first living things were 

 essentially genelike in nature. While we may never know what form life 

 first took, the viruses offer a possible clue, since they have some of the 

 attributes we associate with living things (e.g., reproduction) combined 

 with attributes more characteristic of nonliving material (e.g., the fact that 

 some of them, at least, can be crystallized). It is suggestive that viruses 

 and genes have many points in common: the chemical constituents are 

 similar, nucleic acids playing an essential role; both are capable of repro- 

 duction (self-duplication); both influence the life processes of cells in 

 which they are found; both undergo chemical change (mutation). But 

 whatever may have been the nature of the first living entity, we see in 

 genes the most fundamental units of life — the units basic to homologies of 

 morphology (Chap. 3), of embryology (Chap. 4), of serology (Chap. 6), 

 and of physiology. 



Similarities and Dissimilarities in Metabolism 



The processes of living, such as digestion and assimilation of food, respi- 

 ration, production of energy, excretion, and so on, are included under the 

 general term metabolism. Since plants and animals share so many similari- 

 ties of chemical and physical structure we should expect to find similarities 

 in the living processes in which these like structures participate. The expec- 

 tation is justified, but added to the similarities we find differences no less 

 interesting. 



Turning first to the matter of nutrition we ask: How do living things ob- 

 tain the carbon, hydrogen, oxygen, nitrogen, and so on, to build into pro- 

 teins, carbohydrates, and fats? In the first place, some living things can 

 make use of these elements as they occur in inanimate nature. These are 

 the green plants. If a green plant has available a supply of water (contain- 

 ing hydrogen and oxygen in the same proportions found in carbohydrates), 

 carbon dioxide (in the air), a variety of inorganic salts, and a source of 

 nitrogen such as ammonia or nitrate, it can manufacture its own proteins, 

 carbohydrates, and fats, including all needed vitamins. The wonderful syn- 

 thetic ability of the green substance, chlorophyll, makes this process possi- 

 ble, utilizing energy from the sun. Living things which can thus derive all 

 needed supplies from the inorganic world are called autotrophic. Auto- 

 trophic organisms are absolutely essential to the survival of organisms 

 which lack the ability to manufacture some or all of their proteins, car- 

 bohydrates, and fats from inorganic ingredients. Such dependent organ- 

 isms are called heterotrophic. 



