74 HEREDITY 



first and second generations, less between the second 

 and third, and after a few more years a condition of 

 constancy is reached. It is found that further cross- 

 fertilisation between plants of the same strain does not 

 produce an increase in vigour in the progeny. Evi- 

 dently, then, cross or self-fertilisation as such has no 

 influence on the vigour of the offspring. The loss of 

 vigour is explained as due to the reduction of the 

 strain, at first hybrid with regard to many of its char- 

 acters, to the condition of a pure line. There is evidence 

 that such pure lines are constant, as others are. 



If two such strains are crossed, there is a large and 

 immediate increase in vigour. It is easy to see that 

 the offspring of such a cross may well be more vigorous, 

 on the average, than the original strain propagated 

 continuously by cross-fertilisation. For we now obtain 

 a crop in which all the individuals are hybrid with 

 regard to a large number of characters, whereas in a 

 strain in which crossing is promiscuous the individuals 

 vary greatly with regard to the number of hybrid 

 characters which they contain. In one of ShulTs 

 experiments the average yield of two pure, or almost 

 pure, strains of maize was 29 bushels per acre. The 

 first cross between the two yielded 68 bushels, while 

 the original strain of corn, which had been cross-fertilised 

 continuously for five years, yielded 61^ bushels. 



The reason why the hybrid condition should be con- 

 nected with extra vigour, we do not know. But in 

 practical breeding, where vigour is usually one of the 

 greatest desiderata, the discovery should prove valuable. 



