186 



ISLAND LIFE. 



[part I. 



our doing so. Considering then, that some one or more of the 

 sea-communications here indicated almost certainly existed 

 during Eocene and Miocene times, let us endeavour to estimate 

 the probable effect such communications would have upon the 

 climate of the northern hemisphere. 



The Indian Ocean as a source of Heat in Tertiary times. — 

 If we compare the Indian Ocean with the South Atlantic we 

 shall see that the position and outline of the former are very 

 favourable for the accumulation of a large body of warm water 

 moving northwards. Its southern opening between South 

 Africa and Australia is very wide, and the tendency of the 

 trade-winds would be to concentrate the currents towards 

 its north-western extremity, just where the two great channels 

 above described formed an outlet to the northern seas. As 

 will be shown in our nineteenth chapter, there were probably, 

 during the earlier portion of the Tertiary period at least, several 

 large islands in the space between Madagascar and South India ; 

 but these had wide and deep channels between them, and 

 their effect would probably have been favourable to the con- 

 veyance of heated water northward, by concentrating the 

 currents, and thus producing massive bodies of moving water 

 analogous to the Gulf Stream of the Atlantic.^ Less heat 

 would thus be lost by evaporation and radiation in the tropical 

 zone, and an impulse would be acquired which would carry 

 the warm water into the north polar area. About the same 

 period Australia was divided into two islands, separated by a 

 wide channel in a north and south direction, (see Chapter 

 XXII.), and through this another current would almost certainly 

 set northwards, and be directed to the north-west by the 

 southern extension of Malayan Asia. The more insular con- 

 dition at this period of Australia, India, and North Africa, 

 with the depression and probable fertility of the Central Asiatic 

 plateau, would lead to the Indian Ocean being traversed by 

 regular trade- winds instead of by variable monsoons, and thus 



1 By referring to our map of the Indian Ocean showing the submarine 

 banks indicating ancient islands (Chap. XIX.), it will be evident that the 

 south-east trade winds — then exceptionally powerful — would cause a vast 

 body of water to enter the deep Arabian Sea. 



