128 INBREEDING AND OUTBREEDING 



yield of grain, is smaller. There is also a slight reduction 

 in number of nodes and in rows of grain, but in contrast 

 to the other three characters the change is almost negli- 

 gible. The last two are only slightly affected by environ- 

 mental factors as compared with the others. A plant may 

 be reduced to one-half its normal height by being grown 

 in a poor situation, but the number of nodes will be nearly 

 the same in the two cases. Hence, we see that inbreeding 

 affects plants much in the same way as poor environmen- 

 tal conditions. 



In all of the characters mentioned there is a reduction 

 in variability and change in mean differing in the several 

 lines. This is illustrated in Table IV, in which are given 

 the data for number of rows of grain on the ear of four 

 different plots of the origiual non-inbred variety and 

 four strains derived from this variety after ten genera- 

 tions of self-fertilization. The marked reduction in vari- 

 ability is apparent both in the restricted range of the 

 distribution of the inbred lines compared to the variety, 

 and in the coefficients of variability. 



This reduction in variability applies only to each in- 

 bred line separately. If all the different lines were com- 

 bined together into one population the variation would 

 be greater than that shown by the original material. This 

 is readily apparent from the table ; it also follows from 

 the fact that many characters are produced by inbreed- 

 ing which are seldom seen in the regularly cross- 

 pollinated stock. Inbreeding reduces variability within 

 separate lines, but increases variability in the descend- 

 ants as a whole. 



From the curves on inbreeding given in the preceding 

 chapter (Fig. 24), it was seen that the production of com- 



