674 DISTEIBUTION IN DIFFERENT PARTS OF THE GLOBE. 



5. Indigenous annual plants are extremely rare or absent ; but recently 

 introduced annuals are very abundant in those islets that have been frequented 

 by man. 



The hypotheses advanced to account for the stocking of an oceanic 

 island with plants from a continent are the transport of seeds by 

 currents, winds, or animal agencies, or that these islands in bygone 

 ages formed a part of the continent from which they have now been 

 severed. Hooker looks upon the floras as the remains of plants of 

 an old geological epoch, the congeners of which are seen in the tertiary 

 fossil flora. We have seen that Edward Forbes adopted this view of 

 continental extension and subsequent separation of insular portions. 

 Hooker favours the view of trans-oceanic migration, coupled with 

 Darwin's theory of the derivative origin of species. By this means he 

 accounts for many continental species and genera being represented on 

 an island by similar but not identical species and genera ; for the 

 graduated series of forms extending from variety to genus ; for the 

 absence of whole tribes from the islands ; for the limited floras and 

 the fewness of the species in proportion to the genera. 



Important changes have taken place in the Floras of islands by 

 the agency of man. Thus St. Helena, according to Burchell, had 45 

 indigenous species, of which 40 were peculiar to the island. Now 

 all is changed. The island, when discovered 360 years ago, was 

 wooded. The introduction of goats in 1513 destroyed the vegetation. 

 In 1709 the native ebony (Melhania melanoxylon) still existed, and was 

 used to burn lime. It is now extinct. Plants introduced by General 

 Beatson from Europe, Africa, and Australia, now thrive well. The 

 original native vegetation had its affinity with the Flora of South Africa. 



The regions of the globe, as regards their vegetable productions, 

 are related either in the orders, the genera, or the species of plants which 

 they produce. By Orders (Hinds remarks) the most distant or general 

 resemblances are established, constituting analogy. One family may 

 occupy the place of another in certain regions. Thus, the Mesem- 

 bryacese of South Africa are represented in America by Cactacese ; 

 and in the south of Europe only by a few species of Sempervivum 

 and Sedum. The Ericaceae of the Cape are represented in Australia 

 by Epacridaceae. By Genera, a closer approximation is established — 

 that of affinity. The Cistuses of Spain and Portugal are represented 

 by the Helianthemum of the north of Europe ; and the genera of 

 Abies and Pinus, in arctic and temperate regions, have their repre- 

 sentatives in the genera Araucaria, Ephedra, and Dammara of the 

 south. By Species, the most perfect accordance of characters is 

 established. 



Meyen states that the species of a genus, and genera, and natural 

 orders, proceed from a point, and range themselves round it in concen- 

 tric circles, or spread out from it like rays in all directions ; or are 



