We will now turn to the floras of mountain-summits which are well known to differ from the floras of the neighbouring lowlands. In certain characters, such as dwarfness of stature, hairiness, &c., the species from the most distant mountains frequently resemble each other,--a kind of analogy like that for instance of the succulency of most desert plants. Besides this analogy, Alpine plants present some eminently curious facts in their distribution. In some cases the summits of mountains, although immensely distant from each other, are clothed by the same identical species{362} which are likewise the same with those growing on the likewise very distant Arctic shores. In other cases, although few or none of the species may be actually identical, they are closely related; whilst the plants of the lowland districts surrounding the two mountains in question will be wholly dissimilar. As mountain-summits, as far as their plants are concerned, are islands rising out of an ocean of land in which the Alpine species cannot live, nor across which is there any known means of transport, this fact appears directly opposed to the conclusion which we have come to from considering the general distribution of organisms both on continents and on islands--namely, that the degree of relationship between the inhabitants of two points depends on the completeness and nature of the barriers between those points{363}. I believe, however, this anomalous case admits, as we shall presently see, of some explanation. We might have expected that the flora of a mountain summit would have presented the same relation to the flora of the surrounding lowland country, which any isolated part of a continent does to the whole, or an island does to the mainland, from which it is separated by a rather wide space of sea. This in fact is the case with the plants clothing the summits of _some_ mountains, which mountains it may be observed are particularly isolated; for instance, all the species are peculiar, but they belong to the forms characteristic of the surrounding continent, on the mountains of Caraccas, of Van Dieman's Land and of the Cape of Good Hope{364}. On some other mountains, for instance <in> Tierra del Fuego and in Brazil, some of the plants though distinct species are S. American forms; whilst others are allied to or are identical with the Alpine species of Europe. In islands of which the lowland flora is distinct <from> but allied to that of the nearest continent, the Alpine plants are sometimes (or perhaps mostly) eminently peculiar and distinct{365}; this is the case on Teneriffe, and in a lesser degree even on some of the Mediterranean islands. {362} See _Origin_, Ed. i. p. 365, vi. p. 515. The present discussion was written before the publication of Forbes' celebrated paper on the same subject; see _Life and Letters_, vol. I. p. 88.