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DIVISION OF GENETICS 



HILGARD HALL 



[Reprinted from SCIENCE, N. S., Vol. XXV., tfo. 

 641, Pages 590-591, April IS, 1907.] 



THE rearing of over 20,000 pedigreed speci- 

 mens of Bursa Bursa-pastoris (L.) Britton, 

 has demonstrated the presence of at least four 

 elementary species, all of which breed true 

 when self-fertilized or crossed within the 

 limits of the same elementary form. From 

 over thirty hybrid families the fact is derived 

 that these several elementary forms hybridize 

 in strictly Mendelian fashion, each form 

 which went into the cross coming out again 

 in the perfectly pure extracted dominant or 

 recessive form of the parents. 



The existence of elementary forms in nature 

 within the recognized limits of the species, 

 differing from one another as do the ele- 

 mentary species of Bursa,, in the possession of 

 definite characters that behave as hereditary 

 units, presents a condition that is not unique, 

 but one which has an important bearing upon 

 some of the questions that have been recently 

 discussed. Several atypic plants have ap- 

 peared in the cultures, which have bred true 

 to their atypic characters, when the assump- 

 tion that they were due to chance crosses 

 would have required that they split into the 

 atypic and typic forms in the ratio 3:1. 

 These occurred in families of which the pol- 

 lination was not guarded and their status as 



1 Extract from paper read before the joint meet- 

 ing of Section F and Section G, A. A. A. S., New 

 York City, December , 1906. 



