48 HEREDITY 



general " population " of beans, for large size of seed, 

 a very definite limit would be set to our results ; the 

 ultimate product of selection would be the pure line 

 with the largest average size of seed. A mutation 

 might give us a further increase in size, but selection 

 alone would not produce that steady indefinite increase 

 which it was formerly supposed to cause. The con- 

 stitution of the germ plasm of a pure race is a perfectly 

 definite and constant thing, and not a quantity vary- 

 ing around a mean. These notions have been found to 

 harmonise beautifully with the results obtained in other 

 departments of the subject. 



The principle of the pure line is valuable, not only 

 theoretically, but in the practical improvement of such 

 of our cultivated plants as are habitually self-fertilised. 

 We shall return to this subject when we come to the 

 consideration of the problems of the practical breeder. 



