318 THE EVOLUTION THEORY 



have from the very first a tendency to be purely transmitted, that 

 is, to breed true. 



The facts upon which these views are mainly based are observa- 

 tions on and breeding experiments with a species of evening primrose 

 (Oenothera) which was found in quantities on a fallow potato-field at 

 Hilversum in Holland. It had been cultivated previously in a neigh- 

 bouring garden, and had sown itself thence in the field. The numerous 

 specimens of this (Enothera lamarckiana growing there were in a 

 state of marked ' fluctuating ' variability, but in addition there grew 

 among them two strongly divergent forms which must have arisen 

 from the others, and which led De Vries to bring the parent stock 

 under cultivation, in the hope that it would yield new forms, in the 

 Botanical Gardens at Amsterdam. This hope was fulfilled; in the 

 second cultivated generation there were, among the 15,000 plants, ten 

 which represented two divergent forms, and in the succeeding genera- 

 tions these forms were repeated several times and in many cases, and 

 five other new forms cropped up, most of them in several specimens 

 and in different generations of the original stock. All these new 

 forms, which De Vries calls 'elementary species,' breed true, that is to 

 say, when they are fertilized with their own pollen they yield seed 

 Avhich gives rise to the same ' elementary species.' The differences 

 between tlie new forms are usually manifold, and of the same kind as 

 those between the ' elementary ' species of the wild Linna^an species. 

 But, according to De Vries, what we have been accustomed since the 

 time of Linne to call a ' species ' is a collective category, whose 

 components are these ' elementary ' species which De Vries has 

 observed in liis experiments with Oenothera. In other species, such 

 as Viola tricolor and Draba verna, true-breeding varieties have long 

 been known to botanists, and these have l)een studied carefully and 

 tested experimentally, especially by A. Jordan, and more recently by 

 De Bary. 



All ' species,' according to the Linna^an conception, consist, De 

 Vries maintains, of a larger or smaller number (in Draba there are 

 two hundred) of these ' elementary ' species, and these arise, as is 

 proved by the case of Oenothera, by saltatory or discontinuous ' varia- 

 tions ' which occur periodically and suddenly break up a species into 

 many new species, because the variations of the germ-plasm, which 

 are for a time merely latent, suddenly find expression in the descen- 

 dants of one individual or another. According to this view, species 

 must be the outcome of purely internal causes of development, wliich 

 reveal themselves as ' mutations,' that is as saltatory variations, which 

 are stable and transmissible from the very first, and among which the 



