MEANS AND METHODS OF EVOLUTIONARY CHANGE 339 



on the subject is being dissipated, however, through discoveries of means by 

 which the rate of mutation can be increased. In forms investigated inten- 

 sively mutations have been observed to occur spontaneously at an extremely 

 low but relatively constant rate. For example, a given mutation will occur 

 spontaneously in one of a million individuals of the fruit fly, Drosophila. 

 Muller has discovered that the rate can be greatly increased by bombarding 

 the parents with X rays. Apparently the X rays penetrate to the germ plasm 

 and there induce mutations, both gene mutations and chromosomal aberra- 

 tions. Subsequently, other radiations, such as ultraviolet light, have been 

 found to produce similar eff'ects. The same is true of some chemicals, notably 

 such carcinogenic (cancer-inducing) chemicals as mustard gas. The number 

 of chemicals and physical forces found to induce mutation will undoubtedly 

 increase as investigation continues. 



We should note that present methods of inducing mutations are "shot- 

 gun" methods; the investigator never knows in advance which genes will be 

 afl'ected. He cannot single out a particular gene and cause it to mutate at 

 will. Obviously such an ability would be highly desirable. There is some 

 evidence that antibodies (p. 109) are among the substances capable of in- 

 ducing mutations. Since antibodies have a degree of specificity, they may 

 possibly be more selective in their action than are the radiations and chemi- 

 cals just mentioned. Antibodies formed against lens proteins of the eye, for 

 example, might react selectively with genes concerned with lens formation, 

 causing mutations in those genes only (see discussion of experiments of 

 Guyer and Smith, pp. 344-345 ) . Only the future will disclose whether con- 

 trol of the mutation process in this way can actually be achieved, however. 

 In the meantime it is important to note that radiations, some chemicals, and 

 antibodies form examples of environmental agents which can cause inherita- 

 ble changes in germ plasm. It is to be emphasized that these agents act on 

 the germ plasm directly, not through the intermediacy of body tissues which 

 have been subjected to change. 



We know that our bodies are constantly being subjected to bombardment 

 by radiations in extremely low concentrations — by cosmic rays, for example. 

 Is it possible that such radiations induce "spontaneous" mutations? While 

 there is no reason to doubt that such natural radiations have an effect on 

 the germ plasm, their concentration is so low that they can hardly account 

 for the observed rates at which mutations occur. Other forces must also be 

 involved. 



Another property of mutations is the fact that as far as we know at 

 present they occur in random or haphazard fashion without regard to use- 

 fulness or the needs of the organism. This randomness has caused some 



