94 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



till April are too slight and irregular, especially in tlie neighbour- 

 hood of the Indian coast, to transport any objects from. India to the 

 Mascarene Islands. 



Wallace has suggested that in a stormy area like many parts of 

 the Indian Ocean, small organisms, such as seeds of plants and eggs 

 of invertebrates, may be transported by the winds across seas of 

 considerable breadth, and he supposes that the Azores and some other 

 Atlantic islands have thus been stocked with plants, insects, and 

 mollusca *. In the latter case he especially points out that the 

 efficient transport in this case is not by ordinary winds snch as the 

 trade-winds, for otherwise the Azores would have derived their 

 plants, insects, and shells from America, but by violent gales and 

 storms, which are in the north Atlantic very capricious and irregular 

 in direction. With regard to storms in the Indian Ocean, I con- 

 sulted my brother Mr. H. E. Blanford, who called my attention to 

 the weather- and current-charts already mentioned, and he tells me 

 that no storm in the Indian Ocean ever crosses the Equator, that the 

 storms travel on each side, away from the equinoctial line, and that 

 consequently, as the Mascarene Islands lie south of the Equator, and 

 India to the northward, the transports of seeds or eggs from one to 

 the other by storms is impossible. A good steady wind blows in 

 the S.W. monsoon (May to October) in a somewhat circuitous 

 course from the Mascarene Islands up the African coast, and thence 

 eastward across the Arabian sea ; but this, like the trade-winds of 

 the Atlantic, is not likely to transport solid objects to any distance. 

 The N. E. monsoon in the neighbourhood of the Indian coast is 

 too light and irregular to be of any importance. 



Of course, under exceptional circumstances, light objects might be 

 carried by violent upward currents, such as occur in tropical cyclones, 

 into the higher regions of the atmosphere, as the volcanic dust was 

 carried from Krakatao ; but independently of the fact that the eggs 

 of tropical mollusca and insects would probably be killed by the 

 cold, this mode of transport might explain diffusion throughout the 

 world, but would not account for partial dissemination of special 

 forms confined to certain islands in particular directions. It is true 

 that the difficulty of transport, either by floating objects or by the 

 wind, would be greatly diminished by the presence of large inter- 

 vening islands as already explained ; but still it is doubtful whether 

 the presence of these islands would have any important effect on the 

 winds or currents, so as to obviate the difficulty of transport from 

 India to the Mascarene Islands. 



* Island Life, pp. 247, 251, 253, &c. 



