694 GEOGRAPHICAL A1S T D GEOLOGICAL BOTANY. 



and whence? Speaking generally, insular floras comprise some 

 species which are absolutely peculiar or endemic ; some which 

 are so closely allied to continental forms, that it may readily 

 be conceived that they came from a common stock at a compara- 

 tively recent period ; and, thirdly, species identical with those of 

 ome continent, and that not necessarily the nearest to them, often 

 indeed, not so ; hence the probability that these islets received their 

 vegetation from these distant continents at a geological epoch very 

 remote from the present. Unfortunately cultivation, the destruction 

 of forests, the ravages of goats, rats, and other destructive animals, 

 have in most cases materially altered the character of the flora. 

 The aboriginal flora of St. Helena, for instance, is all but entirely 

 destroyed, and its place supplied by introduced vegetation, or by 

 species in no way peculiar to the island. When near a continent, 

 and divided from it by a shallow sea, the floras of island and con- 

 tinent are, in a broad sense, identical, as those of England and 

 Northern Germany ; but where, as in the case of Madagascar, an 

 island is large and separated by a deep channel several hundreds 

 of miles wide, the flora is different. Some islands are of volcanic 

 origin, like Madeira, the Azores, and the Canaries ; and the anti- 

 quity of their flora can be studied with reference to their geological 

 history. 



The Atlantic Islands. 



The general character of the flora is distinct, with an intermixture of 

 Mediterranean elements, and of species from the African mainland. Lyell 

 considers that these islands had never any connexion with the mainland, 

 but that they originated as volcanoes in Miocene times, and were peopled 

 by waifs and strays from Europe and Northern Africa in that period. The 

 presence of such North- American types as Persea and Clethra is explained 

 by the fact that in Miocene times, when the Atlantic volcanoes first reared 

 their crest above the waves, Europe was covered with a very rich vegeta- 

 tion, containing many genera now peculiar to America (Hooker), so that 

 the genera in question in Madeira may he looked on as "survivals " from 

 the Miocene period. Monizia edults, a native of one of the Desertas, 

 belongs to a genus which has no representative elsewhere in the world. 

 This, too, like Companula Vidalii, which only exists on one rock off the 

 coast of Flores, may be regarded as a " survival." Hooker (Lecture on 

 Insular Floras, < Gardeners' Chronicle,' 1867) considers that Volcanic 

 Islands received their flora by means of immigrants from various con- 

 tinents, and does not favour the view that these distant islets ever formed 

 parts of existing continents. The same>author points out, in general terms, 

 that islands, owing to the similarity of physical circumstances, are peopled 

 with similar plants, and that their vegetation is consequently similar, as 

 in the abundance of Mosses and Ferns and of evergreen trees. Animals, 

 on the other hand, are rare. Species are few in proportion to genera, 

 genera to Orders, hence the remarkable difference in the flora. The 

 mountains, moreover, have relatively few alpine plants. 



