98 INBREEDING AND OUTBBEEDING 



methods which may be carried on with different results. 

 If the individuals are mated at random to all the differ- 

 ent types of parents, homozygous and heterozygous, the 

 effect is the same as in brother and sister mating. Cousin 

 matings, which may proceed either as single or double 

 first-cousin matings or even more distant unions, may be 

 equally or less effective; but all tend towards the same 

 end; heterozygosity is ultimately eliminated and homo- 

 zygosity prevails. In this way there is a difference be- 

 tween selective mating and random mating. Continued 

 selective mating is necessary to bring about homozygos- 

 ity. Intermittent inbreeding alternating with periods 

 of outcrossing which is the prevailing state of affairs 

 with many organisms cannot maintain any high degree 

 of homozygosity. 



In self-fertilization the reduction in heterozygous 

 allelomorphs in a population as a whole follows curve 1 in 

 Fig. 24, irrespective of the number of factors concerned, 

 as stated before, provided that a random sample of all 

 the different classes of individuals are selfed and become 

 progenitors of the next generation and that there is 

 equal productiveness and equal viability. If the hetero- 

 zygotes are more productive, as in many cases they are, 

 the reduction to complete homozygosity will be delayed. 



Artificial self-fertilization in naturally crossed species, 

 then, brings about the same condition as prevails in 

 naturally selfed species. The great variability of a cross- 

 fertilized species gives way to the more uniform and 

 stable condition characteristic of naturally self -fertilized 

 organisms. The uniformity brought about by inbreeding 

 is thus due to a reduction of the genetical variability. 

 Inbreeding affects physiological or developmental vari- 



