86 INTRODUCTION TO EVOLUTION 



press themselves in decreased ability to manufacture needed organic ma- 

 terials from inorganic sources. Apparently abilities possessed by ancestors 

 have been lost by descendants in many cases. If green plants are abundant 

 in the environment, why should an animal manufacture its own carbohy- 

 drates from carbon dioxide and water? The same question might be asked 

 for any other ingredient in the diet. Evidently, then, unneeded capabilities 

 have been lost by organisms. Such a loss would be favored by natural se- 

 lection (Chap. 2), since wasted energy is always disadvantageous. In other 

 words, the losses of synthesizing capability are examples of the trend to- 

 ward specialization which we shall find appearing time after time in our 

 study of evolution. 



How do animals utilize proteins, carbohydrates, and fats contained in 

 their food? Proteins are used in the body to form new tissue, both to in- 

 crease the amount of tissue when the body is growing and to replace worn- 

 out tissue. We obtain proteins from meat, eggs, cereals, and many other 

 foods, but these proteins are not in a form to be used directly by our bodies. 

 They must first be broken down into their constituent amino acids. Then 

 these amino acids are synthesized into the precise proteins needed by the 

 body for growth and repair. The breaking down of proteins into amino 

 acids is one part of the process of digestion occurring in our stomach and 

 small intestine. The process is made possible by the presence in these parts 

 of the digestive tract of substances called enzymes, which act as catalysts. 

 Proteins, being extremely complex chemical substances, are not split into 

 amino acids at one step; a series of changes is involved. Each change in the 

 process is made possible by presence of the appropriate enzyme. The point 

 we wish to make here is that there is great similarity throughout the ani- 

 mal kingdom in these protein-splitting enzymes. 



Carbohydrates and fats are used by the body as fuel, to provide energy 

 for all phases of metabolism, energy for movement and activity, energy in 

 the form of heat (especially in birds and mammals). Some carbohydrates, 

 such as the "simple sugar," glucose, are comprised of small enough mole- 

 cules so that they can pass unaltered through the membrane lining the 

 digestive tract. They are then carried by the blood to the tissues of the 

 body where fuel is needed. But starches, and many sugars, consist of larger 

 molecules which must first be split into "simple sugars" before they can 

 pass through the membrane. As in the case of proteins, this splitting of 

 larger molecules into smaller ones is accomplished by enzyme action. And 

 again there is great similarity throughout the animal kingdom in the en- 

 zymes concerned. The enzymes possessed by an individual species are 

 closely correlated with the food habits of the species. Carnivores (flesh 



