114 INBEBEMNa AND OUTBEEEDING 



may not be interpreted as the result of Mendelian factor 

 recombination; but if one were to base his judgment on 

 them alone, he could not truthfully make the didactic state- 

 ment that inbreeding per se is not injurious. There would 

 ever be the imcertainty with which the additional variable 

 bisexuality always encumbers a genetic experiment. For- 

 tunately, we may turn to the numerous experiments on 

 hermaphroditic plants for the deciding vote. 



Many wild species and cultivated varieties of plants 

 are almost invariably self -fertilized, and apparently lack 

 nothtag in vigor, productiveness or ability to survive. 

 Among wild plants many species of the family Legumi- 

 nossB, among cultivated plants — wheat, rice, barley, oats, 

 tobacco, beans, tomatoes — are types characterized by very 

 nearly continuous self-fertilization, and these plants are 

 in no immediate danger of extinction. 



On the other hand, the majority of the higher plants 

 is provided with devices which promote natural cross- 

 pollination, and show definite injurious effects when in- 

 bred artificially. Even species which are characteristi- 

 cally self-feriilized are crossed occasionally. This, to- 

 gether with the fact that nearly all plants and animals 

 are benefited by crossing, led Knight as early as the close 

 of the eighteenth century to believe self-fertilization is 

 not a natural process and always produces more or less 

 injurious results. His views were summed up in the state- 

 ment, " nature intended that a sexual intercourse should 

 take place between neighboring plants of the same spe- 

 cies." Darwiu, fifty years later, basing his conclusions 

 upon observation of animals and direct experimentation 

 with plants, was even more radical, and concluded that 

 "nature abhors perpetual self-fertilization." 



