HYBRID VIGOR OR HETEROSIS 149 



The frequency distributions of height of plant of the two 

 parents and the reciprocal hybrids are given in Table VI. 



TABLE VI 



HEIGHT OF CROSSES BETWEEN NICOTIANA RUBTICA SCABRA (352) AND 

 N. RUSTICA BRAZILIA (349) 



In both of the first hybrid generations the average height 

 is above the major extreme of either parent. Similar 

 increases in height were obtained when a commercial 

 variety of tobacco was crossed, first with a variety from 

 the same locality, then with one from the opposite side of 

 the world identical with the first in external appearance. 

 On the other hand, strains of tobacco from seed grown in 

 Connecticut when crossed with plants of the same vari- 

 eties from seed grown in Italy showed no increase in 

 vigor. Hence, the mere fact of residence in different 

 parts of the world that is, exposure to different environ- 

 mental conditions has no necessary relation to the 

 phenomenon of hybrid vigor, for such individuals may 

 be alike in constitution. Darwin's repeated emphasis of 

 the good derived from crossing plants whose ancestors 

 were exposed to different conditions was because he 

 thought such differences in environment brought about 

 germinal changes. This attitude, therefore, does not 

 detract from his general position that it is differences 

 in germinal construction which bring about hybrid vigor ; 

 and this is the principal point at issue. 



