HYBRID VIGOR OR HETEROSIS 



243 



normal but very uniform in character (Fig. 144) . But if two 

 of these inbred strains are crossed with each other a great 

 increase in size results in Fi, which as it accompanies restora- 

 tion of the original heterozygosis may reasonably be ascribed 

 to its agency (see Figs. 145-147). If a second generation 

 of the crossed corn is raised by planting seeds taken from Fi 

 plants, there is found to be a falling off in vigor (see Fig. 148). 

 The F2 plants start out well owing to the large amount of 

 food materials stored in the plump Fi seeds, but ultimately 

 they fall behind Fi plants in vigor of growth so that they 

 attain a height considerably less, though still much in excess 



WeigM 



in 

 Grams 



dltaceB. 



Age in Days 40 80 120 160 200 240 280 320 



Fig. 149. Growth curves of race B guinea-pigs and of Cavia cutleri. 



360 



400 



of the inbred parent races. This is in harmony with the 

 view that heterosis is the cause of hybrid vigor, for heterosis 

 should be at a maxunum in Fi and should decline in F2 

 exactly as the height of the maize plants is seen to do in this 

 cross. A case in which ordinary (blending) size inheritance 

 is complicated by heterosis is seen in crosses made between 

 Cavia Cutleri from Peru and races of guinea-pigs which we 

 will call B and C. The growth curve of each of the parent 

 stocks is shown in Fig. 149. In each case males are heavier 

 than females except for the first few weeks of life when the 

 females are heavier. Races B and C are nearly twice as heavy 

 in adult weight as Cavia Cutleri. 



Growth curves of the Fi and F2 hybrids are shown in Figs. 



