Physical Theory of Secular CJianges of Climate. 181 



of North Greenland at the present day ; and were the warm 

 currents of the North Pacific also at the same time stopped, the 

 northern hemisphere would assuredly be under a state of general 

 glaciation. 



Suppose also that the warm currents, havingbeen withdrawn from 

 the northern hemisphere, should flow into the Southern Ocean. 

 What then would be the condition of the southern hemisphere ? 

 This transference of heat from the northern hemisphere to the 

 southern would raise the temperature of the latter hemisphere 

 about as much as it would lower the temperature of the former. 

 It would consequently raise the mean temperature of the antarctic 

 regions much above the freezing-point, and the ice under which 

 those regions are at present buried would, at least to a great ex- 

 tent, disappear. The northern hemisphere, thus deprived of the 

 heat from the equator, would be under a condition of things similar 

 to that which prevailed during the glacial epoch, while the other 

 hemisphere, receiving the heat from the equator, would be under 

 a condition of climate similar to what we know prevailed in the 

 northern hemisphere during a part of the Upper Miocene period, 

 when North Greenland enjoyed a climate as mild as that of 

 England at the present day. 



This is no mere picture of the imagination, no mere hypo- 

 thesis devised to meet a difficult case ; for if what has already 

 been stated be not completely erroneous, all this follows as a ne- 

 cessary consequence from physical principles. If the warm cur- 

 rents of the equatorial regions be all deflected into one hemi- 

 sphere, such must be the condition of things. But how can 

 such a deflection as this take place ? Upon what principles 

 could the warm water of the equatorial region ever have been 

 deflected all, or even nearly all, upon one hemisphere ? What 

 are the physical agencies or conditions that could possibly affect 

 the great currents of the globe to such an extent ? 



The Physical causes which deflect Ocean- currents. — Some of 

 these have been discussed at considerable length on former occa- 

 sions ; but I shall here endeavour to state them in a more brief 

 and concise form. A clear apprehension of them is necessary for 

 the proper understanding of what follows in reference to the in- 

 fluence of ocean- currents on the distribution of plant and animal 

 life on the surface of the globe during geological epochs. 



Secular displacements from the one hemisphere to the other 

 of the great system of ocean -currents take place indirectly from 

 an astronomical cause. The cause to which I refer is the change 

 in the eccentricity of the earth's orbit. Directly, the eccentri- 

 city of the orbit can have no possible influence on ocean-currents ; 

 but indirectly it has, as we shall presently see. Change of ec- 

 centricity, although it cannot directly affect the currents of the 



