CONSANGUINEOUS MARRIAGES 239 



subject enlisted the interest of Mr. Darwin who devoted to it 

 several years of study. By an extensive series of well-planned and 

 controlled experiments Darwin showed that in many plants con- 

 tinued inbreeding was followed by a reduction of the size, vigor 

 and fertility of the stock, and that crosses with related varieties 

 often led to the production of forms with greater vigor than either 

 of the parents. In fact, many plants were found to be sterile when 

 fertilized with their own pollen, although others, such as beans, 

 are regularly self-pollinated. The numerous mechanical and 

 other devices by means of which plants effect cross fertilization, 

 were interpreted as adaptations developed by natural selection 

 for securing the advantages which crossing was supposed to 

 confer. "Nature," says Darwin, "abhors perpetual self-fertili- 

 zation." 



Among animals, cross fertilization is more common than in 

 plants. Male and female sex organs are more frequently borne by 

 separate individuals, but even where hermaphroditism exists, it 

 is an exceedingly rare occurrence for eggs to be fertilized by sperm 

 cells from the same animal. With the exception of some of the 

 Protozoa, we do not meet with that close inbreeding which is 

 found in a considerable number of species of plants. 



"When," says Darwin, "we consider the various facts now 

 given which plainly show that good follows from crossing, and less 

 plainly that evil follows from close interbreeding, and when we 

 bear in mind that throughout the organic world elaborate provi- 

 sion has been made for the occasional union of distinct individuals, 

 the evidence of a great law of nature is, if not proved, at least 

 rendered in the highest degree probable; namely, that the crossing 

 of animals and plants which are not closely related to each other 

 is highly beneficial or even necessary, and that interbreeding 

 prolonged during many generations is highly injurious." 



When we observe the inbreeding of plants and animals we 

 cannot fail to be impressed by the varied results which are found 

 in different forms. In many plants continued self-pollination is 

 followed by rapid deterioration. Shull and also East and Hayes 

 in experimenting with inbred varieties of corn found that there 



