INFLUENCE OF INBREEDING ON HYDATINA SENTA. 7 



The great stature of plant hybrids was noted by Kolreuter 

 (1763); numerous examples were cited by Gartner (1849); and 

 experiments with many plants were recorded by Darwin (1876). 

 The value of crossing was known to Beal (1876) who made recom- 

 mendations in regard to the rearing of corn and other plants of 

 commercial value. More recently there have been a number of 

 discussions, accompanying new evidence from plants. Maize 

 has shown in the hands of G. H. Shull (1908), East (1908), and 

 Collins (1910) that inbreeding is accompanied by deterioration, 

 and that crossing between distinct lines brings about an increase 

 of vigor in FI. East and Hayes (1912) obtain similar results in 

 some crosses of tobacco, though not in all, and Wellington (1912) 

 finds that the yield of tomatoes is increased by hybridization. 



Further citation of such evidence would be superfluous, as 

 rather full bibliographies have been given in recent papers, 

 notably that of East and Hayes (1912). 



MENDELIAN EXPLANATIONS OF VIGOR. 



To explain the cases in which inbreeding is accompanied by 

 deterioration, several theories have been advanced in recent 

 years. Some have suggested that inbreeding greatly increases 

 the chance of producing pure recessive combinations; it is neces- 

 sary to assume also that these recessive characters are bad. But 

 others have pointed out that there is equal chance that indi- 

 viduals homozygous for good qualities may be produced. 



Two other Mendelian explanations have been offered. One 

 was proposed by G. H. Shull (1908) to explain the greater vigor 

 of FI plants of common maize. He found that successive self- 

 fertilization in corn reduced vigor rather rapidly at first and more 

 slowly afterwards, while crossing two unrelated lines resulted in 

 much more vigorous plants in FI. He attributed the vigor of 

 FI to its "hybridity," and the gradual reduction of vigor with self- 

 fertilization to the gradual establishment of the homozygous 

 condition. Whether there is supposed to be a special set of genes 

 for vigor, or whether heterozygosis in ordinary body characters 

 is held responsible for vigor, Shull does not state. Presumably 

 the genes are not all equipotent, so that heterozygosis in one 

 character may contribute more to vigor than heterozygosis in 



