[Chap. XXXIX MUTATIONS 483 



decreased by a change in environment. To illustrate these points we shall 

 have to begin with a simple analogy. 



Glucose, fructose, and mannose are all distinct sugars of the formula 

 CuHiiiOe. If one of them is placed in a bottle of water, the other two 

 will be slowly fomied from it even if the bottle is kept in a uniform 

 environment. But the rate of change from one kind of sugar to another 

 varies with the intensity of certain factors in the environment, such as 

 temperature and the presence or absence of barium hydroxide or other 

 chemicals. Geochemists calculate the age of rocks on the basis of the 

 rate at which uranium changes to lead and helium. Even the rate of 

 change of this process is unaffected by ordinary changes in the environ- 

 ment that occur on the surface of the earth. 



These simple examples are cited as a reminder that within a system 

 (molecule, gene, chromosome, and cell) there may be intrinsic condi- 

 tions that result in change, and that the eflFects of the environment on 

 these intrinsic conditions may result only in altering the rate of change. 

 That is, the effects of the environment may not result in any new com- 

 pound or structure. 



By special laboratory methods chemists are able to make 16 kinds of 

 sugars of the formula C(iHi20(5. Only about one-third of these have ever 

 been found in plants. This fact should keep us from inferring that just 

 anything may happen in a living organism. The number of kinds of 

 changes that may occur in any system is dependent upon conditions 

 within the system. The number that does occur may depend in part upon 

 the environment. Of those that do occur, only certain ones are sufficiently 

 stable to persist. Obviously if all heritable changes in living cells de- 

 pended upon the conversion of one hexose sugar to another, only a few 

 kinds of heritable differences could occur. 



The changes that occur in the composition of the molecules of the 

 genes are not changes in sugar molecules, but changes in the much 

 more complex molecules of proteins, or protein-like substances. The 

 kinds of changes in protein molecules, however, are probably as inde- 

 pendent of changes in the external environment as are those in the mole- 

 cules of sugar. Likewise, the rate of change may be influenced by 

 changes in certain factors of the environment. We should not, therefore, 

 overemphasize environment and underemphasize the intrinsic nature of 

 change in hereditary units of matter. If differences in environment are 

 important in causing heritable change, one would expect that artificial 



