Duplicate genes for capsule-form in Bursa bursa-pastoris. 117 



obviously correct inference were it not for the fact, already mentioned, 

 that our cultural treatment of Bursa has become much more successful. 

 Elimination after germination has been almost completely conquered; 

 at any rate it has been reduced until it has become a nearly negligible 

 factor. The fact that these later generations have given a nearly 

 normal distribution of the ratios may consequently be taken as strongly 

 supporting the hypothesis that the deficiency of Heegeri plants in the 

 earlier cultures was due to selective elimination. The cause for the 

 elimination was probably not, however, as was assumed, the constitu- 

 tional weakness of the Heegeri type, but only its longer period of vege- 

 tative development. There has been in general an excess of the bursa- 

 pastoris type among the plants first coming to bloom and a corresponding 

 excess of Heegeri among the plants last to bloom. This relation will 

 l>e considered more in detail on another occasion. It need only be 

 pointed out here that if cultures which have stood for a long time in 

 the greenhouse become unhealthy from the attacks of insects or fungous 

 pests, or from the cumulative effects of any unfavorable conditions of 

 the environment, the more slowly developing plants will suffer most; 

 and if many die from such causes without having fruited, there would 

 result just the differential elimination necessary to explain the deficiency 

 of Heegeri plants in the injured families. 



While at first sight this seems to be an adequate explanation 'of 

 the deficient ratios, there are indications that this may not be the whole 

 story. The elimination of a disproportionate number of Heegeri plants 

 should have no influence on the composition of the bursa-pastoris portion 

 of the same family. In the case of a monohybrid family this would 

 mean that one-third of the bursa-pastoris plants would be homozygous 

 and two-thirds heterozygous, regardless of the deficiency in the number 

 of recessives present. The only monohybrid family (No. (59284) in the 

 Fs from the original cross, consisted of 42 bursa-pastoris and 9 Heegeri 

 or 4'67 : 1. An attempt to test the constitution of all the bursa-pastoris 

 individuals in this family by selfing them, was successful in the case 

 of 39 of them, and these 39 were shown by their progenies (Table IV) 

 to have consisted of 8 homozygotes and 31 heterozygotes, instead of 

 the 13 homozygotes and 26 heterozygotes that were to be expected. 

 It is thus seen that this F 3 family (No. 09284) really presented a ratio 

 approximately 1:4:1 instead of 1:2:1. 



The fact that the deficiency in the recessive class is ^balanced 

 by a similar deficiency in the number of homozygous dominants may 

 be merely a coincidence, of course, this possibility being rendered 



