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IV. The cause of the variation. 



The multitudinous different isoreagents (geno- 

 typically conditioned variations, microspecies) within 

 the collective species Viola tricolor L. have arisen from 

 crosses between V. tricolor and V. arvensis and between 

 the segregation products from these crosses. 



In the paper from 1921 I proposed this theory, and I am 

 now able to confirm it. The evidences are threefold: 



1. Analytical evidence. 



Seeds of piants from populations with both arvensis, tricolor 

 and the intermediate types give by sowing not only the mother- 

 type but also the other combinations. This is characteristic for 

 hybrids. An instance will show this clearly: 



In 1921 I collected seeds from the population F mentioned 

 in the paper from 1921 (p. 210 — ^211, table I and II), a stone- 



Table III. 



