302 BROOKLYN BOTANIC GARDEN MEMOIRS 



species of the pine or lily, the chromosomes are always of the same 

 number and do not vary as in the examples cited above, as occurring 

 in connection with hybridism. Frequently the addition of chromo- 

 somes under hybrid conditions is not simply a doubling, tripling, etc., 

 of the original number but a mere arithmetical addition. Among the 

 vascular cryptogams and the mosses similar cases of multiplication 

 of the normal number of chromosomes are frequently found in species 

 growing in very damp situations or actually in the water and in which 

 hybridization is accordingly favored. 



Having enumerated a few of the morphological characteristics of 

 hybrids we are now in the position to apply the facts elucidated to the 

 case of the Oenotheras, which have been brought particularly into 

 prominence in connection with the mutation hypothesis of De Vries. 

 In the so-called mutants of Oenothera lamarckiana as well as in that 

 species itself, we find all the cited stigmata of hybridism as presented 

 by incompatible species, namely a high degree of sterility, amounting 

 in some cases to complete impotency, particularly in the male gametic 

 cells, failure to segregate in accordance with Mendelian ratios and the 

 multiplication of the number of chromosomes beyond the normal 

 gametophytic number seven, or sporophytic fourteen. In 0. gigas 

 the gametophytic chromosomes are 14 instead of the normal seven, 

 while in 0. semigigas there are 21 in the sporophyte instead of the 

 normal fourteen and in 0. lata, 0. semilata and 0. rubricalyx, fifteen. 

 Similar conditions have been described in many other species of 

 Oenothera and their so-called mutants. It accordingly appears 

 abundantly clear when morphological considerations are brought into 

 court as well as the physiological data derived from experimental 

 breeding that the genus Oenothera is obviously affected by contamina- 

 tion through hybridization in its various species and their so-called 

 mutants. The conclusion may accordingly be drawn that so far as 

 the genus Oenothera throws light upon the origin of new species at all, 

 it vouches rather for the multiplication of species as a consequence of 

 hybridization than for their appearance as a result of the mysterious 

 process of mutation. 



In conclusion we may turn to an objection which has been raised 

 by De Vries and other mutationists to the interpretation of morpho- 

 logical sterility as an evidence of hybridization. It has been claimed 

 that this feature is an accompaniment of mutation. It is most un- 

 fortunate for the mutationists that a phenomenon so generally recog- 

 nized as associated with the crossing of species should at the same 

 time occur in mutating forms. The burden of proof that such forms 

 are not of hybrid origin distinctly lies with the mutationists. We 

 have however positive evidence that this is not a possible interpre- 



