PLANT BREEDING AND EVOLUTION 



179 



both wild and cultivated species of plants and animals. A 

 great majority of cultivated species apparently profit by cross- 

 pollination, and many species actually deteriorate if continually 

 self-pollinated. Common field corn has been shown by the 

 experiments of East and Shull to belong to this class of plants. 

 These investigators selected and isolated two strains of corn 

 (Fig. 91), which were then close-pollinated for several generations. 

 The result was, in 

 each case, a greatly 

 weakened race, with 

 the ears reduced to 

 nubbins. But when 

 these two weakened 

 races were crossed, 

 offspring with much 

 increased vigor and 

 productiveness were 

 secured. Any such 

 first-generation cross 

 will always split up 

 in succeeding gener- 

 ations into a variety 

 of forms in which the 

 characters of the par- 

 ents will be differ- 

 ently combined, and 

 the importance of such crosses to agriculture and horticulture is 

 therefore somewhat diminished. In other species, as in Darwin's 

 experiment with peas, no advantage arises from crossing, and 

 the plants are evidently adapted to continuous self-pollination 

 and self-fertilization. Among the species which are known to 

 be adapted to continuous self-pollination, without detrimental 

 effects, are many of the cereals, including wheat, barley, and oats, 

 as well as peas, soy beans, potatoes, tomatoes, flax, and tobacco. 

 Darwin's pioneer experiments were, nevertheless, of funda- 

 mental importance in emphasizing the value of cross-pollination 

 and cross-fertilization. 



FIG. 92. Increased yield of first-generation 

 crosses in corn 



Showing two ears of Tyler's White-Cap Dent at the 

 left and of Burwell's Yellow Flint at the right. The 

 first-generation cross of these two varieties (repre- 

 sented by the two ears in the center) has consistently 

 yielded more than the dent, the higher-yielding parent. 

 Photograph furnished hy the Connecticut Agricultural 

 Experiment Station 



