CHAP. XIX THE MADAGASCAR GROUP 412 



extinction of some of the types that were originally 

 preserved, and may leave the ancient fauna in a very 

 fragmentary state ; while subsequent elevations may have 

 brought it so near to the continent that some immigration 

 even of mammalia may have taken place. If these 

 elevations and subsidences occurred several times over, 

 though never to such an extent as again to unite the 

 island with the continent, it is evident that a very 

 complex result might be produced ; for besides the relics 

 of the ancient fauna, we might have successive immigra- 

 tions from surrounding lands reaching down to the era of 

 existing species. Bearing in mind these possible changes, 

 we shall generally be able to arrive at a fair conjectural 

 solution of the phenomena of distribution presented by 

 these ancient islands. 



Undoubtedly the most interesting of such islands, and 

 that which exhibits their chief peculiarities in the greatest 

 perfection, is Madagascar, and we shall therefore enter 

 somewhat fully into its biological and physical history. 



Physical Features of Madagascar, — This great island is 

 situated about 250 miles from the east coast of Africa, and 

 extends from 12° to 25|^° S. Lat. It is almost exactly 

 1,000 miles long, with an extreme width of 360 and an 

 average width of more than 260 miles. A lofty granitic 

 plateau, from eighty to 160 miles wide and from 3,000 to 

 5,000 feet high, occupies its central portion, on which rise 

 peaks and domes of basalt and granite to a height of 

 nearly 9,000 feet ; and there are also numerous extinct 

 volcanic cones and craters. All round the island, but 

 especially developed on the south and west, are plains of a 

 few hundred feet elevation, formed of rocks which are 

 shown by their fossils to be of Jurassic age, or at all events 

 to belong to somewhere near the middle portion of the 

 Secondary period. The higher granitic plateau consists of 

 bare undulating moors, while the lower Secondary plains 

 are more or less wooded ; and there is here also a con- 

 tinuous belt of dense forest, varying from six or eight to 

 fifty miles wide, encircling the whole island, usually at 

 about thirty miles distance from the coast but in the 

 north-east coming down to the sea-shore. 



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