304 ANNTJAL REPORT SMITHSONIAN INSTITUTION, 1955 



the expected composition of the population. These deviations may 

 sometimes chance to occur in the same direction, like a run of luck, 

 until the hereditary composition of the population is quite altered. 



Human control over the mutation process began in 1927 and 1928 

 when my former teacher H. J. IVIuller and my later friend and mentor 

 L. J. Stadler, working quite independently, the one with fruit flies and 

 the other with maize and barley, succeeded in demonstrating that ex- 

 posure to X-rays enormously increases the frequency of all kinds of 

 mutations. Other kinds of potent radiations, and even ultraviolet 

 rays, were found to do the same. During the course of World War II, 

 as a byproduct of the scientific study of poison gases, chemical com- 

 pounds were discovered that likewise enhance the frequency of muta- 

 tion. Dozens of workers are now actively studying the conditions that 

 limit or enhance the action of physical and chemical mutagens. From 

 all this exploration there is arising the ability — not yet to direct the 

 course of mutation so as to produce just the mutation desired, or even 

 a particular type of mutation, for that is still impossible — to increase 

 the over-all frequency of mutation so that once-rare hereditary changes 

 become common. 



Most of the mutations produced are harmful to their carriers, as 

 might be expected from a blind interference with the delicately bal- 

 anced mechanisms of life. Most mutants have a lower viability and a 

 poorer fecundity than the types they are derived from. Yet this is 

 not always so. Sometimes a new mutant type may be poorer than the 

 original type under the existing conditions of life, but may prove 

 itself superior when these are altered. Flies dependent on garbage 

 pails do better in the city of Baltimore if they have wings, but on the 

 storm-swept island of Kerguelen in the southern Indian Ocean the 

 only flies to be found creep about without wings, or with little stubby 

 vestiges of wings. Natural selection, as Darwin pointed out, deter- 

 mines the differential survival of various hereditary types, and natural 

 selection is but a name for the complex combination of conditions 

 under which each population lives and reproduces, and which is dif- 

 ferent, at least somewhat different, in every other time and place. 



For a long time now mankind has substituted for the selection of 

 nature his own artificial selection of whatever chance mutations ap- 

 peared in his domestic animals and cultivated plants and which seemed 

 to him to be desirable. It is thus that all the progress in plant and 

 animal breeding has been made, from the day the first animal was 

 tamed and the first seed planted to the beginning of our own century. 

 AVhat geneticists are now enabled to do is merely to speed up this 

 process a thousandfold and to control and direct it more effectively. 

 Thus, for example, it was discovered about five years ago that certain 

 inbred strains of field corn have as high a sugar content in the stalk 

 as sugarcane itself. The genetics has been worked out, selection has 



