328 EVOLUTION, GENETICS, AND EUGENICS 



ready been shown in previous chapters, hundreds of new hereditary 

 types arose, apparently spontaneously, in pure-pedigreed stock. Each 

 new type is designated a mutant, and the cause of the changed heredi- 

 tary condition is not a gross chromosomal change, but an invisible 

 change at a definite point in a definite chromosome, whose cause is 

 unknown but whose location can be exactly determined. Such muta- 

 tions are known as gene mutations. Like the mutants of Oenothera, 

 these Drosophila mutants do not differ from the parent species in just 

 one or two characters, but in several or many characters. Usually some 

 one or two characters in any given mutant are especially character- 

 istic, and these serve to give a name to each mutant and make it easier 

 to identify them. Both morphological and physiological characters 

 are involved in these mutants, and every part of the body may be 

 involved. Sometimes the change is so slight as to require an eye 

 sensitized by much training to detect them. It may happen, for ex- 

 ample, that two mutants of the eye are so much alike that the human 

 eye is not sufficiently keen to tell them apart, but they may be dis- 

 tinguished by differences in their hereditary behavior. A large per- 

 centage of the mutants discovered in Drosophila are "lethals," which 

 means that the change is decidedly for the worse, under the prevailing 

 conditions of life, and that they render the individual unfit to live. 

 Possibly under decidedly different conditions some of these lethal 

 mutants might be better adapted than the normal individuals. A 

 further discussion of the role of mutants in evolution will be given in a 

 later connection. 



The following two rather technical, but very interesting, discus- 

 sions as to the nature, causes, and significance of mutations are from 

 two men who are recognized as perhaps the leaders in the two branches 

 of mutation study. R. R. Gates has done a large amount of important 

 work especially upon the cytology of Oenothera, and H. J. Muller has 

 done and is still doing much to enrich our understanding of the muta- 

 tional phenomena exhibited by Drosophila. While these two workers 

 do not agree as to the relative emphasis that should be placed upon the 

 two types of mutation, it is obvious that both types are of very great 

 importance in evolution. 



THE NEO-MUTATIONIST POSITION 

 R. RUGGLES GATES 



Since the original work of De Vries, the subject of mutation in 

 Oenothera has advanced in many directions and the explanation of the 

 phenomena has taken on various aspects. Mutation has become 



