19 1 5] DE VRIES— OENOTHERA BIENNIS 179 



by actual crosses between different elementary types, such as we 

 ordinarily suppose to occur in polymorphic groups in nature. 



The other alternative, that it is logically inconceivable that 

 hybridism could have presided at the origin of new species, coin- 

 cides 'exactly with the current conception of the mutability in the 

 evening primroses. New forms originate through the evolution 

 of new characters, as in O. gigas and O. ruhricalyx;^'' or through 

 the loss of existing ones, as in O. nanella and 0. rubrinervis; or by 

 means of the appearance of qualities, which were probably latent 

 in the parent race, as in 0. lata and 0. scintillansJ^ These cases 

 are evidently not recombinations of existing characters. If it is 

 conceded that the hypothesis of a hybrid origin does not apply to 

 them, it is obviously unimportant for the theory whether or not, 

 besides them, there are other instances which may be considered 

 as hybrid recombinations. 0. semigigas, which is a hybrid between 

 a normal and a mutated sexual cell, has never been considered as 

 an argument against the mutation theory. 



In cultures of chrysomelid beetles, W. L. Tower has observed 

 hereditary changes which run almost parallel to the mutations of 

 0. Lamarckiana. He started from crosses between Leptinoiarsa 

 decemlineata, L. multiiaeniata, and L. oblongata, and obtained 

 constant races. When given proper treatment by changing their 

 environic factors, these races could be made to break up, and they 

 did so in a manner at least partially analogous to that of the 

 evening primroses." 



It is obvious that the fact that mutations may be artificially 

 induced in hybrid strains does not contradict the contention that 

 they may arise in pure strains also. But from the experiments of 

 Tower it seems that some hybrid strains at least are more liable 

 to show the phenomenon. 



'' 0. gigas is considered to be a progressive mutant on account of its double number 

 of chromosomes and its special behavior in crosses. 0. rubricalyx, which arose in the 

 cultures of Gates from rubrinervis, and which I cultivated this summer from seeds 

 kindly supplied by him, is perhaps the most beautiful among all the mutants of 

 0. Lamarckiana. Its red color is something quite new in the group. It behaves as a 

 Mendelian dominant in crosses with its parent species and is therefore obviously of a pro- 

 gressive nature; see Gates, R. R., Amer. Nat. 43:600. 1911. 



■' See Gruppenweise Artbildung. Berlin, pp. 244-260. 1913. 



'» Tower, W. L., Evolution of the chrysomeUd beetles. Carnegie Institution of 

 Washington Yearbook no. 12:68-71. pi. 3. 1913- 



