

FACTORS OF INHERITANCE. 45 



Johannsen took a number of beaiis from a large field, and 

 found that the offspring of each differed to some extent 

 from that of the others in the position of the mean weight. 

 While the whole population of beans varied between 

 20 and 90 centigrams, the offspring of one bean might 

 vary between 20 and 65 centigrams with a mean of about 

 50, and the offspring of another between 40 and 90 with 

 a mean of about 60 centigrams. So the curve of varia- 

 tion of the whole population is made up of a number 

 of curves, each of which has its own mean, each of whick 

 corresponds to a strain derived from one beau. Each 

 strain, with its own inheritance giving a characteristic 

 mean round which fluctuation may take place, is called 

 a " pure line." Inheritance is* uniform within each pure 

 line, so that light or heavy beans taken from the same 

 line will yield offspring varying about the mean charac- 

 teristic of the line. A bean weighing 55 centigrams 

 might belong to either of the pure lines mentioned above ; 

 to which of them it belongs would only be revealed by the 

 average weight of the beans derived from it. 



If the various strains making up a population were 

 really pure, of uniform inheritance, then under uniform 

 conditions they should yield, not an even curve, but a 

 series of steps there would be a sudden jump from one 

 strain to the other. It is, therefore, the modifications 

 which smooth down these steps to a graduated curve 

 corresponding to the variation of all the strains com- 

 bined. 



Pure lines can rarely if ever persist in nature, except 

 perhaps in such forms as Bacteria, where sexual reproduc- 

 tion is not known to occur. Even here inheritance must 

 sooner or later split into new strains owing to some altera- 

 tion in its factors. But practically all organisms repro- 

 duce sexually ; there is a perpetual crossing and mixing 

 of strains, self-fertilisation being quite abnormal. With 

 these few exceptions, factors of inheritance are brought 

 into the zygote by the gametes of two parents. What 



