164 Changing Plant and Animal Nature 



have not been confirmed, and it is possible that the muta- 

 tions, or heritable traits departing from ancestral types, hap- 

 pened to appear in the course of the experiments and were 

 really unrelated to the artificial conditions imposed upon the 

 developing parental beetles. 



More recently, Professor H. J. Muller, of the University 

 of Texas, has succeeded in inducing mutations in fruit flies 

 by exposing the insects to graded doses of radium before 

 maturity. The adult individuals thus treated did not show 

 the effects of the radiation; but among their offspring there 

 appeared large numbers of individuals with one or more 

 departures from the standard pattern of the wild red-eyed 

 fruit fly. There were several distinct alterations of the wings, 

 of the eyes, of the abdomen and of other parts — over one 

 hundred of the same kinds of sports as had previously ap- 

 peared spontaneously in the cultures of Dr. Thomas H. Mor- 

 gan and other students. The radium had apparently done 

 something to the germ cells of the male and female flies, some- 

 thing that influenced the development of subsequent gen- 

 erations and not merely the immediate progeny. The aber- 

 rant traits were all heritable (see page 272). The effects of 

 the radium seem to be no different in kind from those of 

 other forces normally acting upon the species; but the radia- 

 tions seem to accelerate the normal processes several hun- 

 dred times. 



Multiple Influences 



The distribution of variations about a mean or ** aver- 

 age point " suggests the operation of many factors. We can 

 see this if we imagine that each individual in the course of 

 attaining his full stature, for example, can be influenced 

 by only two factors, with the chances even of meeting either, 

 neither or both. There would accordingly be four classes 

 of such individuals: the very tall ones, that had met with 

 the favorable factor; the very short ones that had met with 

 the unfavorable factor; the medium-sized individuals that 



