2o6 HEREDITY AND EUGENICS 



But, on the other hand, inbreeding by no means 

 necessaril}^ leads to deterioration. Among plants, 

 garden beans (Phaseolus vulgaris) and the small- 

 flowered evening primroses (of which there are many 

 species) are regularly self-pollinated, and probably 

 are crossed only at long intervals under exceptional 

 circumstances. Yet they are not lacking in vigour, 

 and, indeed, the small-flowered (Enotheras are much 

 more widespread in their wild condition in North 

 America than the large-flowered forms which are 

 open-pollinated, and hence give greater chances for 

 crossing. The former have been more successful in 

 an evolutionar}- w^ay, despite their self-pollination. 

 The advantages of ensured seed production for 

 every flower have more than counterbalanced any 

 advantages derived from crossing. 



In this connection it is necessary to remember 

 several points. First, the h^^brid vigour or heterosis 

 arising from crossing both in plants and in animals 

 is confined very largely, or in some cases entirely, to 

 the first hybrid generation. It is therefore a purety 

 temporary phenomenon, unless the crossing is con- 

 tinued in each generation. It is apparently not even 

 a constant accompaniment of the heteroz3^gous con- 

 dition, but is speciall}^ characteristic of the F^ 

 generation. Another point to remember is that in- 

 breeding, which must always occur to some extent 

 even amongst wild animal species, tends to reduce 

 the amount of heterozygosity in each generation. 

 Probably in man, in whom a host of character- 

 differences are being redistributed in each generation, 

 a homoz3^gous condition is seldom reached, except as 

 regards a relatively small number of characters. 

 The degree to which the homozygous condition exists 

 in any family may be to some extent measured by 

 the amount of diversity between sibs (brothers and 

 sisters) and their parents, grandparents, and collateral 



