Parallel Mutations. 45 



the type of biennis only in having pale yellow petals. It is found 

 occasionally on the sand dunes and must have originated as a 

 mutation. 



The well-known uniformity and constancy of (E. biennis, 

 together with the apparent impossibility of crossing in the regions 

 from which these plants were derived, greatly enhances the 

 importance of the mutations obtained from them. This species, the 

 only one with which Linnaeus was at first acquainted, was originally 

 brought from North America in 1614 the first evening primrose 

 to be brought to Europe. 1 It was soon naturalized on the Dutch 

 sand dunes, where it has retained its characters with remarkable 

 uniformity for three centuries until the present time. Davis (1914) 

 admits that no species in cultivation has greater claims to be 

 regarded as a pure species, and at one time was willing to admit 

 that if mutations were produced by it they would go far to prove 

 the correctness of the mutation hypothesis that the production of 

 new forms is not merely a phenomenon of hybridity. Indeed it 

 would be difficult to find any species whose claims to "purity" or 

 freedom from crossing are better authenticated. Not only is it 

 self-pollinated, so that crossing would be a rare event at best, but 

 for three centuries in these Dutch localities it has been more or 

 less completely isolated from its American relatives. Even if its 

 ancestors were crossed before they left America, 300 generations 

 of self-fertilization would be more than ample to -reduce it to a 

 homozygous condition again, 2 unless a condition of balanced lethal 

 factors maintained it in a heterozygous state. 



But the Drosophila work shows (Muller, 1918) that such a 

 condition of balanced lethals is not the result of crossing, but of 

 alterations which arise in the germ-plasm. Another fact against 

 the hybridity theory is this. The mutant sulfurea, which has been 

 shown to arise occasionally in cultures, is also " far from rare " on 

 the Dutch dunes (de Vries, 1915b). Apparently it was first 

 mentioned in 1687 by Hermannus in Hort. Acad. Lugduno-Batavi 



1 For the early history of some of these forms see Mut. Factor, p. 47. 



1 Without dealing with the mathematical formulae for inbreeding involved 

 (see East and Jones, 1919, p. 91), it may be pointed out that even if a large 

 number of independent differences were involved in an original cross, the 

 population from self-pollination would be practically all homozygous by the 

 tenth generation. The population would then be expected to contain a 

 number of different homozygous types. It might be anticipated that the 

 weaker of such types would fail in the struggle for existence. But if we 

 examine the facts, we find that CE. biennis in Holland is remarkably uniform 

 and has always been so, never showing any variations on the dunes except 

 the sulfurea mutation, and leptomeres with cruciate petals. There is thus a 

 complete absence of data on which to found a hypothesis of hybridity, while 

 the evidence to the contrary is strong. 



