138 INBREEDING AND OUTBREEDING 



gun; it is likely, therefore, to vary directly with the 

 amount of cross-breeding experienced by their immediate 

 ancestors. The rapidity of the isolation of homozygous 

 types is a function of the intensity of the inbreeding. 



Take the case of maize as an example. Maize is one 

 of the most variable of cultivated plants, and is usually 

 cross-pollinated under natural conditions. In other 

 words, the individuals making up any commercial variety 

 of maize are each and every one heterozygous for a large 

 number of hereditary factors a heterozygosis that is 

 kept up by continual crossing and recrossing. When such 

 a variety is inbred there is automatic isolation of homo- 

 zygous combinations, following simple mathematical laws 

 as we have already seen. If self-fertilization is practiced, 

 stabilization through an approximately complete homo- 

 zygosis occurs after a relatively small number of genera- 

 tions ; if a less intense system of inbreeding is followed, 

 the result is the same, but it is obtained more slowly. Dur- 

 ing thisi process, before stabilization is reached, there is 

 reduction in size, vigor and productiveness following 

 somewhat roughly the reduction in per cent, of hetero- 

 zygousness. We can think of this reduction in vigor as a 

 change correlated with approaching homozygosis if we 

 wish, although as we shall see there is reason to believe it 

 to be a result of linked inheritance. What does occur is 

 a reduction in vigor of the population as a whole in each 

 generation associated with the isolation of individuals 

 more homozygous than their parents. Any particular in- 

 dividual may be vigorous or weak, fertile or sterile, nor- 

 mal or monstrous, good, bad or indifferent, depending 

 wholly upon the combination of characters received. 

 Many of the characters which become homozygous will be 



