1874.] Climate of the Glacial Period. 431 
endeavouring to show that other physical causes would be 
brought into operation during a great ellipticity of the 
orbit which would tend to decrease the temperature of the 
hemisphere that had its winter in aphelion, and to increase 
that of the other. The most powerful of these he con- 
siders would be a change in the great currents of the 
ocean by which at present a large amount of heat is con- 
veyed from the tropics to the poles. He maintains that 
these currents are produced by the trade-winds, and that 
when the temperature of one hemisphere was reduced and 
the other increased in the manner and by the causes already 
discussed, the trade-winds on one side of the equator would 
be weakened and on the other strengthened, and in con- 
sequence the warm currents flowing towards the poles 
would in one hemisphere be augmented and in the other 
decreased, if not stopped altogether. For instance, he 
considers that the Gulf Stream is produced by the action of 
the trade-winds, and that in case of a great ellipticity of 
the orbit when the winter of the northern hemisphere 
happened in aphelion the air would be chilled, whilst that 
of the southern hemisphere would be warmed, and thus 
the aérial currents flowing from the poles towards the 
equator would be altered. Under these circumstances 
“the winds from the severe wintry north would sweep with 
much more vigour towards the equator than the opposite 
winds from the south pole. And hence Mr. Croll contends 
that with weaker winds blowing from the south the great 
antarctic drift-currents would be reduced in volume, while 
the subsidiary currents to which they give rise, namely, the 
broad equatorialand the Gulf Stream, would likewise lose in 
volume and force. And to such an extent would this be 
the case that, supposing the outline of the continents to 
remain unchanged, not only would the Brazilian branch of 
the equatorial current go on at the expense of the Gulf 
Stream, but the Gulf Stream he thinks would eventually be 
stopped, and the whole vast body of warm water that now 
flows north be entirely deflected into the southern ocean.’* 
Well may Mr. Geikie say that the effect of the withdrawal 
from the north of all these great ocean rivers of heated 
water would be something enormous. 
But is Mr. Croll’s theory of the origin of the Gulf Stream 
correct? Is it possible to believe that the great. body of 
water in the Atlantic Basin would be warmed at one end 
and cooled at the other without some system of circulation 
* The Great Ice Age, p. 142. 
