CHAP. XIX.] 



THE MADAGASCAR GROUP. 



417 



The abundance of orchids may be in part due to analogous 

 causes. Their usually minute and abundant seeds would be as 

 easily carried by the wind as the spores of ferns, and their 

 frequent epiphytic habit affords them an endless variety of 

 stations on which to vegetate, and at the same time removes 

 them in a great measure from the competition of other plants. 

 When, therefore, the climate is sufficiently moist and equable, 

 and there is a luxuriant forest vegetation, we may expect to 

 find orchids abundant on such tropical islands as are not too 

 far removed from other lands or continents from which their 

 seeds might be conveyed. 



Concluding remarks on Madagascar and the Mascarene Islands, 

 — There is probably no portion of the globe that contains within 

 itself so many and such varied features of interest connected with 

 geographical distribution, or which so well illustrates the mode 

 of solving the problems it presents, as the comparatively small 

 insular region which comprises the great island of Madagascar 

 and the smaller islands and island-groups which immediately 

 surround it. In Madagascar we have a continental island of 

 the first rank, and undoubtedly of immense antiquity; we 

 have detached fragments of this island in the Comoros and 

 Aldabra ; in the Seychelles we have the fragments of another 

 very ancient island, which may perhaps never have been 

 continental ; in Mauritius, Bourbon, and E-odriguez we have 

 three undoubtedly oceanic islands ; while in the extensive banks 

 and coral reefs of Cargados, Saya de Malha, the Chagos, a,nd the 

 Maldive Isles, we have indications of the submergence of many 

 large islands w^hich may have aided in the transmission of organ- 

 isms from the Indian Peninsula. But between and around all 

 these islands we have depths of 2,500 fathoms and upwards, 

 which renders it very improbable that there has ever been here 

 a continuous land surface, at all events during the Tertiary or 

 Secondary periods of geology. 



It is most interesting and satisfactory to find that this conclu- 

 sion, arrived at solely by a study of the form of the sea-bottom 

 and the general principle of oceanic permanence, is fully sup- 

 ported by the evidence of the organic productions of the several 

 islands ; because it gives us confidence in those principles, and 



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