130 INBREEDING AND OUTBREEDING 



pletely homozygous types by self-fertilization is greatest 

 in the generations from the third to the sixth if a large 

 number of factor differences are involved at the start. 

 The experimental results obtained from these inbred 

 strains of maize fit this theory well. It is not until after 

 about three generations of self-fertilization that extreme 

 types begin to appear. While there has been a reduction 

 in size and productiveness before this, it is at this time, 

 or during the next two or three generations, that the 

 greatest diversity of types occurs. It is here that most 

 of the monstrosities and plants which are unable to re- 

 produce themselves appear. 



From Table IV we see that equally striking changes 

 in the mean row number also take place. The averages 

 have been shifted both up and down from the original 

 conditions. The greatest segregation has taken place 

 between the first and the eighth generations. In the eighth 

 generation the lines were again split up, but show no 

 marked change after this point. Differences in the ears 

 of these inbred strains of corn are shown in Fig. 29. 



The rate of reduction in variability and rate of change 

 of mean are shown by the data for row number of two 

 of the inbred strains for successive years in Table V and 

 Fig. 30. These two lines are descended from the same 

 plant in the second year of self-fertilization. The figures 

 previous to the third year are not available, and in that 

 year only for one of the strains, but since then a marked 

 change in average row number, and a reduction in vari- 

 ability have taken place without conscious selection one 

 way or the other. Though the number of plants grown in 

 the generations from the 7th to the 10th are too few to be 

 a basis for accurate conclusions, the sharp increase in 



