54 BURSA BURSA-PASTORIS AND BURSA HEEGERI : 



appears that B. dp. heteris is much more generally distributed than any of 

 the others, and this favors the view that this is the original type from which 

 B. bp. rhomboidea, tenuis, and simplex were derived by retrogressive muta- 

 tion. Recessiveness of the last three to the first is probably rarely if ever 

 any advantage to them, however, since all of these forms appear to be 

 about equally adapted to the range of habitats in which they grow. 



Mendelian inheritance also has an important bearing upon the distribu- 

 tion of the various biotypes of a species; for the transportation of a single 

 hybrid seed may carry all of the biotypes which are related to one another 

 in the Mendelian way, however many there may be,* and two pure-bred 

 seeds landed in the same vicinity may lead to the same result. Thus any 

 seed of B. bp. heteris which has been produced by pollination with pollen 

 from B. bp. simplex, or vice versa, or any seed of B. bp. rhomb oidea that 

 has resulted from pollination with pollen from B. bp. tennis, or vice versa, 

 will give rise to a progeny in the F 2 which will include all four of these 

 biotypes, and self -evidently two pure-bred seeds representing either of 

 these two pairs of elementary species will carry the capacity to produce the 

 same four types in the third generation from the time they find themselves 

 in juxtaposition in a new locality. 



The same principle would hold if there were 3 pairs of Mendelian char- 

 acters involved, but then 8 biotypes might be carried by a single hybrid seed. 

 This situation would be realized by the material dealt with in this paper 

 if a single seed of B. bp. simplex pollinated by B. heegeri, or vice versa, 

 were taken to a new locality. Transeau (1907) has pointed out how on 

 the same principle any number of biotypes might be introduced by means 

 of a single pollen grain into a new locality where a single biotype had 

 existed before, and to which heavy seeds might find much greater diffi- 

 culty in being transported. 



Still another important effect of Mendelian inheritance in the promotion 

 of organic evolution is brought out by my crosses between B. bursa-pastoris 

 and B. heegeri, namely, the production of parallel series of biotypes in 

 nearly related species. Up to the time this cross was made B. heegeri was 

 known only in the heteris form, but among the hybrid offspring were 4 

 distinct pure-breeding biotypes of B. heegeri. It is thus seen that the 

 single mutation by which B. heegeri originated from B. bursa-pastoris 

 doubled the number of possible biotypes of Burs a in the world, provided 

 all such other biotypes behave as do the four under discussion in this paper. 



It may be added that the facts here shown that the rosette of B. heeget ; 

 represents the same Mendelian units that are present in B. bursa-pastoris 

 and that there is only a single fundamental difference between these two 

 species is the best possible proof of the origin of B. heegeri from B. bursa- 



*Except in the presence of "spurious allelomorphism." 



