Fitness in the Living World 3 2 l 



although certain organic compounds can now be made arti- 

 ficially in the laboratory, every living thing performs many 

 complicated chemical processes which the chemist can neither 

 duplicate nor understand. In particular the power which 

 all kinds of protoplasm have of converting food substances 

 into their own peculiar kinds of protoplasm the power of 

 assimilation is a chemical secret which the mind of man 

 has not been able to discover, although every cell of his body 

 knows this secret. The secret of "fixing" free nitrogen was 

 discovered by some of the simplest bacteria hundreds of 

 millions of years ago, and their efficiency in this respect is 

 much greater than man can hope to attain either at Niagara 

 Falls or Muscle Shoals. The ability of all green plants to 

 convert water and carbon dioxide into sugar and starch in 

 the presence of sunlight is a secret of such importance that 

 if man could duplicate the process cheaply and efficiently 

 it would forever solve the problem of the food supply. The 

 chemical processes involved in fermentation and digestion 

 may be artificially duplicated by man, but only with the aid 

 of chemical substances known as enzymes, which are made 

 by even the simplest kinds of protoplasm but which cannot 

 be artificially produced by man. 



Other substances known as chemical messengers, or hor- 

 mones, which are produced by certain ductless glands and 

 which circulate in the blood, profoundly influence the 

 growth, development, and activity of many distant parts of 

 the body indeed, many of the most remarkable correla- 

 tions of growth and form, of function and structure, of dif- 

 ferentiation and integration, are determined by hormones. 

 There is good reason to believe that they are the real ma- 

 terials of heredity, and that they determine race, sex, and 

 type of personality; but although these hormones may be 

 produced by chromosomes, cytoplasm, and glands, they can- 



