228 GENETICS AND EUGENICS 



we call self-pollination, in which the egg-cells of the plant are 

 fertilized by pollen-cells produced by the same individual. A 

 similar phenomenon occurs among some of the lower animals, 

 notably among parasites. But in all the higher animals, 

 including the domesticated ones, such a thing is impossible 

 because of the separateness of the sexes. For here no indi- 

 vidual produces both eggs and sperm. The nearest possible 

 approach to self-pollination is in such cases the mating of 

 brother with sister, or of parent with child. But this is less 

 close inbreeding than occurs in self-pollination, for the 

 individuals mated are not in this case identical zygotes, 

 though they may be similar ones. 



It has long been known that in many plants self-pollina- 

 tion is habitual and is attended by no recognizable ill effects. 

 This fortunate circumstance allowed Mendel to make his 

 remarkable discovery by studies of garden peas, in which the 

 flower is regularly self-fertilized, and never opens at all unless 

 made to do so by some outside agency. Self-pollination is 

 also the rule in wheat, oats, and the majority of the other 

 cereal crops, the most important economically of cultivated 

 plants. Crossing can in such plants be brought about only 

 by a difficult technical process, so completely adapted is the 

 plant to seH-pollination. And crossing, too, in such plants is 

 of no particular benefit, unless by it one desires to secure 

 new combinations of unit-characters. 



In maize, or Indian corn, however, among the cereals, the 

 case is quite different. Here enforced self-pollination results 

 in small unproductive plants, lacking in vigor. But racial 

 vigor is fully restored by a cross between two depauperate, 

 unproductive individuals obtained by self-fertilization, as has 

 been shown by Shull. This result is entirely in harmony 

 with those obtained by Darwin, who showed by long-con- 

 tinued and elaborate experiments that while some plants do 

 not habitually cross and are not even benefited by crossing, 

 yet in many other plants crossing results in more vigorous 

 and more productive offspring; that further, the advantage 

 of crossing in such cases has resulted in the evolution in 



