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 
