EVOLUTION CON KLIN 215 



These mutations seem to go in all possible directions, but not in all 

 directions. Most of these mutant flies are less viable than the wild 

 stock from which they came, and many are lethal, that is, they kill 

 their possessor sooner or later, but a few of them are progressive. 

 They may occur in germ cells or in somatic cells. In short, wherever 

 there are genes these may undergo mutation. The fact that most of 

 these mutations are degressive rather than progressive has led some 

 persons to doubt whether they can be the materials for evolution, 

 but it is necessary to remember that much evolution has been degres- 

 sive, and the small number of progressive mutants as compared with 

 the multitude of regressive ones teaches us at what a price progress 

 has been bought. 



IV 



The nature of the changes in genes by which mutations are caused 

 is unknown, but it seems probable that it is some kind of physical 

 or chemical change. The fact that it may affect one gene and not 

 another similar one that is not more than one-thousandth of a milli- 

 meter away would seem to indicate that it is not some general en- 

 vironmental influence. This consideration led Muller (1927) to the 

 conclusion that it might be due to some form of radiation similar to 

 those by which physicists knock electrons out of atoms. Consequently 

 he subjected Drosophila to X-rays and found that the frequency of 

 mutation was increased about 150 times. Some of these mutants 

 were of the same type as were previously known, but many were 

 new. Most of them were detrimental, and more than half of them 

 were lethals, but some of them were carried through 50 generations 

 without reverting. In addition to gene mutations. X-rays cause 

 breaks and translocations in chromosomes, which in turn cause 

 marked changes in the developed animals. 



A similar increase in mutation has been caused by X-rays in the 

 case of barley, corn, the jimpson weed, a wasp, Habrobracon^ et at. 

 They have also been induced by radium and possibly by cosmic 

 rays. But mutations are far too common and X-rays and radium 

 far too uncommon to warrant the conclusion that mutations are 

 generally caused by these means. 



Searching for some more common cause of mutation Goldschmidt 

 (1929) found that by heating the eggs of Drosophila to such a degree 

 as to kill most of them he obtained from the survivors two new 

 types, and Jollos reports (1930, 1931) that larvae of Drosophila that 

 were subjected to a temperature of 36° C. for 15 to 23 hours pro- 

 duced during 8 months more than 100 mutants, while not a single 

 one appeared in his controls. Most important of all, some of these 

 mutations were " orthogenetic " or progressive in a definite direction, 

 the body color, for example, increasing from " sooty " to " ebony " 



