Similarly in another exceedingly important group of plants, the Cycadaceæ[139], the examples usually met with in botanical gardens are quite insufficient as standards of comparison when we are dealing with fossil forms. Familiarity with a few commoner types leads us to regard them as typical for the whole family. In Mesozoic times cycadean plants were far more numerous and widely distributed than at the present time, and to adequately study the numerous fossil examples we need as thorough an acquaintance as possible with the comparatively small number of surviving genera and species. The less common and more isolated species of an existing family may often be of far greater importance to the palaeobotanist than the common and more typical forms. This importance of rare and little known types will be more fully illustrated in the chapters dealing with the Cycadaceæ and other plant groups. Among Dicotyledons, the Natural Order Proteaceæ, at present characteristic of South Africa and Australia, and also represented in South America and the Pacific Islands, is of considerable interest to the student of fossil Angiosperms. In a valuable address delivered before the Linnean Society[140] in 1870 Bentham drew attention to the marked ‘protean’ character of the members of this family. He laid special stress on this particular division of the Dicotyledons in view of certain far-reaching conclusions, which had been based on the occurrence in different parts of Europe of fossil leaves supposed to be those of Proteaceous genera[141]. Speaking of detached leaves, Bentham says:—“I do not know of a single one which, in outline or venation, is exclusively characteristic of the order, or of any one of its genera.” Species of _Grevillea_, _Hakea_ and a few other genera are more or less familiar in plant houses, but the leaf-forms illustrated by the commoner members of the family convey no idea of the enormous variation which is met with not only in the family as a whole, but in the different species of the same genus. The striking diversity of leaf within the limits of a single genus will be dealt with more fully in volume II. under the head of Fossil Dicotyledons. [Sidenote: VENATION CHARACTERS.]