410 



INSULAR FLOEAS AND FAUNAS WITH 



[Ch. XLI, 



are doubtless tlie remnants of a flora derived from an ancient 

 and adjoining Miocene 



continent. 



must 



Miocene 



rradn- 



ally gave place to another of Pliocene date, and all these 



^^ ^^ ^^ rV 



fluctuations in the animate world must have made themselves 

 felt in the oceanic islands in which the successive destruction 

 and renovation of the terrestrial surfaces would facilitate the 

 settling in them of new species brought to them by the 

 winds, marine currents, and various agents of transport, or- 

 ganic and inorganic. New sheets of lava would in particular 

 weaken the barrier which preoccupancy opposes to new colo- 

 nists ; for the melted matter first annihilates every living 

 thing over the strip of land, more or less broad, which extends 

 from the volcanic orifice to the sea-coast, and then, after many 

 years, when the lava has decomposed, it aflbrds a fresh and 

 virgin soil on which new immigrants may settle. Volcanic 

 ejections and movements of upheavaJ, by causing perpetual 

 variations in the surface-level of each island above the sea, 

 would also promote fluctuations in the fauna and flora. Tha 

 low portion of Africa which is marked in our map (fig. 138, 

 p. 407), as the Sahara, was probably under water during the 

 Miocene period. It is also possible that some volcanic islands 

 may, during or since the Miocene era, have been formed and 

 again destroyed within the area embraced in this map. They 

 may have played an important part in promoting the inter- 

 change of species between diff^erent archipelagos, or between 

 them and the continent. 



It will be seen that at present, about half way between 

 Madeira and the Canaries, there are some isolated rocks 

 called the Salvages, which attain a height of 100 feet above 



i. 



the sea. The 



them 



mile 



iD 



'om 



much 



in size by the waves. The plants, insects, and landshells 

 found upon them belong in part to those peculiar types called 



Miocene 



remarks 



the Atlantic islands are indispensable to a reader who would 



manner 



