Elementary Species in Nature 53 



ula, Spergula media and salina and many other 

 pairs of allied species have differentiating char- 

 acters of the same value as those of the element- 

 ary species of Drab a verna, Filago, Plant ago, 

 Carex, Ficaria and a long series of other genera 

 afford proofs of the same close relation be- 

 tween smaller and larger groups of species. The 

 European frost-weeds or Helianthemum in- 

 clude a group of species which are so closely al- 

 lied, that ordinary botanical descriptions are 

 not adequate to give any idea of their differen- 

 tiating features. It is almost impossible to 

 determine them by means of the common ana- 

 lytical keys. They have to be gathered from 

 their various native localities and cultivated 

 side by side in the garden to bring out their 

 differences. Among the species of France, ac- 

 cording to Jordan, Helianthemum poUfoUum, 

 H. apennimim, H. pilosum and H. pulverulen- 

 tum are of this character. 



A species of cinquefoil, Potentilla Tormen- 

 tilla, which is distinguished by its quaternate 

 flowers, occurs in Holland in two distinct types, 

 which have proved constant in my cultural exper- 

 iments. One of them has broad petals, meeting 

 together at the edges, and constituting a round- 

 ed saucer without breaks. The other has nar- 

 row petals, which are strikingly separated from 

 one another and show the sepals between them. 



