CROSSING AND VIGOR OF OFFSPRING 565 
the loss of vigor in the offspring. Hybrids are usually more 
vigorous than their parents. Corn grown from seed resulting 
from self-fertilization shows much loss in vigor and consequently 
does not yield so well (Figs. 486 and 487). The difference in 
yield between plants resulting from crossing and plants resulting 
from self-fertilization often amounts to several bushels per acre. 
Fic. 488. — Increase in size of fruits in Cucumbers as a result of crossing. 
a and c show size of fruits borne by the parents and 5, the size of fruits borne 
by the first generation of the cross. After Halsted. 
Darwin found that Cabbage plants obtained by crossing were 
nearly three times the weight of those obtained by self-fertiliza- 
tion. In Buckwheat Darwin obtained plants much taller and 
about one-fifth better in yield by crossing. In Lettuce, Beets, 
Pumpkins, Squashes, Tomatoes, and many other plants (Fig. 
488), it has been shown that crossing produces more vigorous 
offspring. 
