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 Rodriguez we have 
three undoubtedly oceanic islands ; while in the extensive banks 
and coral reefs of Cargados, Saya de Malha, the Chagos, and the 
Maidive Isles, we have indications of the submergence of many 
large islands which 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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