56 BURSA BURSA-PASTORIS AND BURSA HEEGERI : 



the ratio 9 : 3 : 3 : 1, in association with each type of capsule. The ratio 

 of the bursa-pastoris type of capsule to the heegeri type is about 22 : 1. 



(6) Bursa heegeri possesses the same unit-characteristics in the rosette 

 as B. bp. heteris, which serves to further confirm its direct derivation from 

 that species. 



(7) The fact that the various forms of Bursa show Mendelian inherit- 

 ance on crossing- is conclusive proof that they are distinct biotypes, not- 

 withstanding- the fact that their distinguishing characteristics are readily 

 rendered latent by fluctuation. 



(8) In the presence of Mendelian inheritance the swamping- of a new 

 characteristic does not take place, and the kind of variation which has been 

 supposed to be swamped by crossing- is just the kind that is most certain to 

 be preserved. 



(9) The recessiveness of a newly arisen form is to be considered an 

 advantag-e in its struggle with the parent, whenever the former is in any 

 way less adapted to its environment than the latter. This principle appar- 

 ently has no bearing upon these biotypes of Bursa, however, as all appear 

 to be equally adapted to the rang-e of habitats in which they live. 



(10) Bursa bursa-pastoris heteris appears to have a more g-eneral distri- 

 bution than any of the other forms. This is probably the primitive type 

 from which the other biotypes have been derived by retrogressive mutation. 



(11) Mendelian inheritance also assists in the distribution of the vari- 

 ous biotypes, since a single hybrid seed or two pure-bred seeds may carry 

 into a new locality as many distinct biotypes as are related to each other 

 by Mendelian characters. A single pollen-grain may do the same provided 

 one biotype is already present in the locality in question. 



(12) Crosses between nearly related species may give rise to parallel 

 series of biotypes in the two species concerned. Before my crosses between 

 Bursa heegeri and B. bursa-pastoris were made, the former was known only 

 in 1 form, but out of the crosses came 4 distinct biotypes of B. heegeri, 

 corresponding with the 4 biotypes which have been demonstrated in B. 

 bursa-pastoris . 



(13) Each mutation which results in the appearance of a new Mende- 

 lian unit-character doubles the possible number of Mendelian biotypes 

 belonging to the species in question, however numerous they may be 

 already, except as limited by "spurious allelomorphism." 



STATION FOR EXPERIMENTAL EVOLUTION, July 11, 1908. 



