No. 539] ORIGIN OF SPECIES IN NATURE 643 



flower, due to a variation in the relative amounts of 

 chloroplasts and yellow chromoplasts, the plant appears 

 to have undergone a sufficient number of fairly rigid 

 tests, under widely divergent conditions, and for a suffi- 

 cient number of generations (9), to entitle it to recogni- 

 tion as a distinct, non-pathological variety. The varia- 

 tion in the color of the flower I am inclined to ascribe to 

 differences in light intensity. In a bed, situated in the 

 middle of the experiment garden, i. e., in an open place 

 where no obstruction prevents full and direct illumina- 

 tion, the flowers are uniformly green. When the plants 

 are grown in the greenhouse or in the shade of shrubs, 

 there seems to be a tendency for the flowers to assume a 

 more yellow tinge. But not in a single instance has a 

 flower been observed which possibly could have been mis- 

 taken for one of the species. 



Again it may be that a form deserving specific rank is 

 discovered, as instanced by the now well-known Capsella 

 Heegeri. 1 Such discoveries possess an added charm since 

 in these cases the possibility of a recent origin of the 

 new form is not excluded. Sometimes it is possible to 

 prove this experimentally, as was done by de Vries for 

 his evening primroses. An illustration of probably 

 recent origin and of repetition of mutation is yielded by 

 the discovery of a single specimen of the inermis variety 

 of Cynara Cardunculus in Algiers by Trabut. 8 But some 

 years ago what would seem to have been the same variety 

 was grown in the St. Louis experiment garden from seed 

 which my father kindly obtained for me from the then 

 director of the Botanical Garden at Buenos Aires and 

 the ancestry of which I understand was to be traced to an 

 individual growing wild in the vicinity. 9 



