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 . 1 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. 
