30 THE FAMILY AND THE NATION 



are appearing, our material ceases to be homogeneous, 

 and the normal curve gives no indication of the actual 

 distribution. 



Light is thrown on the distinction between the two 

 kinds of variation by the recent work of Johannsen on 

 what he calls " pure lines." The weights of the seeds 

 of a single variety of bean differ from each other. 

 Most seeds have weights near the mean value, but a 

 few diverge more or less from it, in accordance with 

 the usual normal curve. Plants, raised from the 

 heavier seed and self- fertilized, themselves produce 

 seed the average weight of which is in general high, 

 while the light seed produces usually light -seeded 

 offspring. 



But, if a single seed be taken and made the starting- 

 point of a family of self-fertilized descendants, no agree- 

 ment, even of the general kind noted above, is found 

 between the weight of a seed in the family and the 

 seeds of the individuals which grow from it. The 

 variations in weight of the seeds have no hereditary 

 value — they are not transmitted. 



Such a family, derived from a single original ances- 

 tral seed, is called by Johannsen a " pure line." All 

 individuals within the family have the same ancestry, 

 and are therefore genetically similar. The variations 

 which they show are then accidental, and are not trans- 

 mitted in heredity. To variations of this type the 

 name " fluctuation " has been given. 



It seems, then, that individuals with an identical 

 ancestry of this particular kind form a stable type. 

 The individuals may vary more or less, but they cannot 

 transmit their variations to their offspring. The offspring 



