THE FINAL ADJUSTMENTS 125 



on in living cells are carried out with the aid of en- 

 zymes, and these enzymes seem always to be proteins. 

 There are, of course, many other classes of compounds 

 in living cells, for instance, lipids (i.e. fats and oils of 

 various kinds), carbohydrates and many other complex 

 organic substances; but there are many reasons which 

 make it seem reasonable to suppose that all these sub- 

 stances are secondary consequences of the actions of 

 protein enzymes. This cannot yet be definitely proved, 

 but most biologists at present adopt as a working hy- 

 pothesis the idea that the primary action of a gene is 

 to cause the production of some particular protein 

 which may act as an enzyme and therefore lead even- 

 tually to the elaboration of various other types of sub- 

 stance of the kinds just mentioned. 



In the last few years a great deal of the attention of 

 people interested in development has been concen- 

 trated on trying to understand the mechanisms by 

 which genes are able to control the formation of par- 

 ticular proteins. Very great advances have been made 

 in this, and we can now say that we have at least an 

 outline of the answer. Although the problem is essen- 

 tially an embryological one, the answer to it has not 

 been found, in the main, by studies on embryos, but 

 by a converging attack along a number of lines which 

 at first sight do not seem at all closely related to em- 

 bryonic development. Some major contributions, for 

 instance, have come from the study of heredity in 

 micro-organisms such as bacteria and viruses. Another 

 important line of evidence has been the X-ray crystal- 



