How Mutations Are Brought About 395 



old and the new differing in an " almost imperceptible de- 

 gree." For example, we do not need to assume that the 

 evolution of the zebra's coat pattern was attained in the 

 course of many generations by the addition of one stripe at 

 a time: the stripe pattern could have appeared at one step, 

 by a change in a single factor or gene. 



It has long been known that animals exposed to the 

 action of X-rays may sustain injuries not only in the body 

 tissues but also in the reproductive organs. Such exposure 

 sometimes results in sterility, and under certain circum- 

 stances in the production of abnormal progeny. Systematic 

 experiments with mice have shown definitely that the chro- 

 mosomes of the germ cells are influenced by X-rays acting 

 upon the body of either the male or female parent; and that 

 the abnormalities in the progeny are accompanied by similar 

 disturbances in the chromosomes of the body cells. There 

 have been several series of experiments in which the action 

 of alcohol upon the parent was followed for several genera- 

 tions with results that indicated some injury to the germ 

 plasm. There is also some evidence that lead absorbed into 

 the body may, without poisoning the parent, influence the 

 germ cells and give rise to abnormal offspring. 



These modifications of germinal material through ac- 

 tion upon the parent have been described as " racial poison- 

 ing," although their precise operation is not understood. 

 Many lines of experimentation are under way in the attempt 

 to clear up the problem, since the influence which the 

 modified body may exert upon the germ plasm is an im- 

 portant consideration in connection with the problem of the 

 transmission of acquired characters. The positive evidence 

 available may be taken to show that the germ plasm is not 

 completely shielded against influences that act upon the 

 body. It does not, however, warrant us interpreting the 

 facts as pointing to the transmission of parental acquire- 

 ments, since in these cases the effect manifested by the 

 offspring is totally different from the effect manifested bv 

 the parent. For example, intoxication of the father leads 



