In other genera such nearly allied species are more or less universally recognized. _Galium Mollugo_ has been divided into _G. elatum_ with a long and weak stem, and _G. erectum_ with shorter and erect stems; _Cochlearia danica_, _anglica_ and _officinalis_ are so nearly allied as to be hardly distinguishable. _Sagina apetala_ and _patula_, [53] _Spergula media_ and _salina_ and many other pairs of allied species have differentiating characters of the same value as those of the elementary species of _Draba verna_. _Filago_, _Plantago_, _Carex_, _Ficaria_ and a long series of other genera afford proofs of the same close relation between smaller and larger groups of species. The European frost-weeds or _Helianthemum_ include a group of species which are so closely allied, that ordinary botanical descriptions are not adequate to give any idea of their differentiating features. It is almost impossible to determine them by means of the common analytical 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, according to Jordan, _Helianthemum polifolium_, _H. apenninum_, _H. pilosum_ and _H. pulverulentum_ are of this character. A species of cinquefoil, _Potentilla Tormentilla_, which is distinguished by its quaternate flowers, occurs in Holland in two distinct types, which have proved constant in my cultural experiments. One of them has, broad petals, meeting together at the edges, and constituting rounded saucer without breaks. The other has narrow petals, which are strikingly separated from one another and show the sepals between them. [54] In the same manner bluebells vary in the size and shape of the corolla, which may be wide or narrow, bell-shaped or conical, with the tips turned downwards, sidewards or backwards. As a rule all of the more striking elementary types have been described by local botanists under distinct specific names, while they are thrown together into the larger systematic species by other authors, who study the distribution of plants over larger portions of the world. Everything depends on the point of view taken. Large floras require large species. But the study of local floras yields the best results if the many forms of the region are distinguished and described as completely as possible. And the easiest way is to give to each of them a specific name. If two or more elementary species are united in the same district, they are often treated in this way, but if each region had its own type of some given species, commonly the part is taken for the whole, and the sundry forms are described under the same name, without further distinctions.