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THE AMERICAN NATURALIST [Vol. XLV 



hardly likely that many could regard this complexity as 

 an invention of Mendelians. The latter would only too 

 gladly have the facts as simple as possible. 



There have remained, however, several instances in 

 which hybrids apparently did not segregate in the F 2 

 generation. Mendel himself investigated one such case, 

 the genus Hieracium. The investigation of Ostenfeld 2 

 made this case perfectly clear by showing that the hy- 

 brids reproduced apogamously. Such asexual reproduc- 

 tion may also explain the behavior of hybrids between 

 species of brambles which are also said to breed true in 

 all their characters. These cases, however, and others 

 among animals of which human skin color is the example 

 par excellence, may be left out of consideration because 

 no exact data concerning them have been forthcoming. 

 There remain the experiments of two careful investi- 

 gators who observed no segregation in the F 2 generations 

 of their hybrids, those of Lock 3 upon heights of maize 

 plants and those of Castle 4 upon weights and ear lengths 

 of rabbits. Lock expected that if segregation occurred 

 it would be into two classes, i. e., simple mono-hybridism. 

 For this reason he made no measurements which would 

 show whether he obtained the kind of segregation which 

 as is shown later in this paper, does occur in maize hy- 

 brids. Castle 5 has recently admitted the possibility that 

 his numbers were not large enough to prove definitely 

 that segregation involving several small unit characters 

 does not occur in the ear length of rabbits. 



The difficulty attending this earlier work was that there 

 was no way of explaining different manifestations of the 

 same character. Segregating characters could always be 

 interpreted either as the presence and absence of a unit 



4 Ostenfeld, C. H., 1904, "Zur Kenntnis der Apogamie in der Gattung 

 Hieracium," Ber. Dcutsch. Bot. Ges., 22: 7. 



"Lock, R. H., 1906, " Studies in Plant Breeding in the Tropics," HI, 

 Experiments with Maize, Ann. Boy. Bot. Gard. Peradeniya, 2 : 95-184. 



* Castle, W. E., et al., 1909, "Studies of Inheritance in Rabbits," Car- 

 negie Inst. Wash. Pub., 114: 5-70. 



6 In lectures at the Lowell Institute, Boston, Mass., 1910. 



