— 409 — 



When the existing and not yet existing but possible hereditary 

 different types are as many as in V. arvensis — tricolor, the selection 

 has a large material to aet upon, and by and by, eHminates the 

 least fit types. 



The West-Jutland dune types are in the character parvi- 

 foliata recessive to the common grandifoliata types. Likely the 

 parvifoliata originally came into existence by a mutation in a 

 place far from the dunes. This character is not profitable in piaces, 

 where the transpiration is normal. But as the factor for it is 

 recessive, it can ■"emigrate concealed" (Raunkiær 1920, pag. 4) 

 heterozygotic, even if all the homozygotic parvifoliate individuals 

 die before they can disperse seeds. When it reaches to a locality, 

 where the character induced by this factor is profitable, it becomes 

 homozygotic by elimination of botli the heterozygotic and homo- 

 zygotic grandifoliate individuals. If A is the factor for grandifol 

 and a the factor for parvifol the circumstances might be illustrated 

 by the following diagram: 



aa cannot live in the grass field owing to the strong com- 

 petition there. Aa and AA cannot live in the dunes because of 

 the strong transpiration there. Therefore these two isoreagents 

 do not intrude on each others territories. Even if they cross at 

 the boundaries, the type not fit to the conditions is ehminated. 

 Only in this way the aforementioned peculiar circumstances at 

 Holmsland Klit (pag. 375, fig. 5 a and b) can be explained. 



Stabilization of conditions in a place will certainly, by and 

 by, cause the isoreagents fit to the conditions at the spot in ques- 

 tion, to become more and more homozygotic; those with the 

 recessive characters first, but also those with the favourable 

 dominant characters, as all the recessive here have gradually 

 been eliminated when they are segregated. The heterozygotes 

 themselves decrease in proportion to the dominant homozygotes 

 unless the first should be better fit to live than the latter. Accord- 



