582 THE PROBABLE ORIGIN OF OENOTHERA LAMARCK lANA SER. 



might be produced by crossing immutable species was a most 

 unhappy one. 



The second condition for success in this kind of work is, as has 

 been stated, the purity of the types to be crossed. As already 

 quoted, Davis assumes that a cross between two very distinct 

 evolutionary lines may give a hybrid with marked modifications of 

 germinal constitution. This may be applied to his choice of the 

 type which he calls 0. grandiflora and which he has made the 

 other parent of his initial cross. He got his seeds from Dixie 

 Landing, Alabama, a locality where Bartram had discovered 

 0. grandiflora about a century ago. He assumed them to be of the 

 pure species, but a culture which I made in my garden from seeds 

 kindly supplied to me by Mr. Davis proved to be a mixture and 

 thereby threw a distinct doubt upon the purity of the station. 

 For this reason I visited Dixie Landing in September 1912 and had 

 the good fortune to be accompanied by Mr. H. H. Bartlett, of 

 Washington, well known for his systematic researches among 

 the wild species of this group. We found the station in a most 

 desolate condition. A small-flowered species, 0. Tracyi, in almost 

 all respects different from 0. grandiflora, had migrated into the same 

 old cotton fields and mixed everywhere with the species of Bar- 

 tram. i) On no single field was the original form pure; it was always 

 mixed to such a degree with 0. Tracyi and with their hybrids that 

 we found it impossible to collect undoubtedly pure grandiflora 

 seed from this locality. Moreover, the intermediate types were 

 so numerous (over a dozen) that it was difficult to regard all of them 

 as normal hybrids between only two parents. To produce such 

 a diversity of forms, either one or both of the parents must have 

 been in a mutating condition, or more than two species must have 

 combined in the crosses. In both cases, the material can hardly 

 be considered as a fit starting-point for experiments bearing upon 

 the causal relations of crossing and mutability. 



Recently I have shown that besides 0. biennis some other species 

 of Oenothera are actually in a state of mutability, and especially 

 has one of the most common American types thrown off marked 

 mutants in my experiment garden. 2) The degrees of development 

 of this condition, however, are very different in different species. 

 In some of them mutations occur rarely, but they serve to throw 



i) The evening primroses of Dixie Landing, Alabama. Op^a VI, 



P- 551- 



2) Gruppenweise Artbildung, pp. 2q6 — 312, 1913. 



