THE MUTATION THEORY 503 



condition, we do not know very much about the causes of this kind of 

 mutation. 



We are also almost entirely in the dark as to the causes of gene 

 mutations. In Drosophila the hundreds of mutations seem to occur 

 over and over again under highly standardized environmental condi- 

 tions. Moreover, attempts to increase the rate or the character of 

 mutations by radical changes of the environment have given nega- 

 tive results. The experiments of Tower described in chapter xxvii 

 stood for a long time as the only instance of the successful production 

 of mutations under experimental conditions. Even these experiments 

 that have been the chief reliance of the environmentalist now seem to 

 be untenable, for it has not been possible to get any confirmation of 

 Tower's results. Using the same apparatus and the same stock, no 

 such mutants as he described, nor any other certain mutants, appeared. 

 It seems likely that Tower happened to get a strain of beetles that were 

 mutating of their own accord and that their mutations happened to 

 coincide with the experiment. MacDougal's experiments, cited in the 

 same chapter, now seem to be far from satisfactory as evidence that 

 true mutations may be induced by rather gross experimental means. 

 Apart from the fact that the percentage of changed individuals was 

 very small, although a great many experiments were performed, it is 

 now reported that the effects faded out in subsequent generations of 

 progeny; and this, of course, would mean that the changed condition 

 should be called an induction, not a mutation. 



The most promising attack upon the problem of the causes of muta- 

 tions has been made by Guyer and Smith, by Stockard, by Bagg and 

 Little, and a few others, who have succeeded in reaching the germ cells 

 with agents from without that seem to be capable of producing per- 

 manent or hereditary changes. Of these, Guyer and Smith are the 

 only ones who appear to have produced anything like a specific change 

 by means of a specific agent. It will be recalled that these investi- 

 gators produced defects of the eye, particularly of the lens, by injecting 

 anti-lens serum into the mother at a time when the lenses of the 

 fetuses were undergoing differentiation, and that the induced defect 

 became definitely hereditary. Guyer also reports that the same results 

 were obtained by needling the lens of the mother, thus inducing directly 

 the production of lens antibodies that seem to be inherited. It has 

 already been pointed out that similar eye defects have been induced 

 by non-specific agents, such as alcohol and X-rays, and that these 

 conditions are inherited in similar fashion. In addition to eye defects 



