SIR J. W. LUBBOCK ON THE EARTH'S AXIS. 399 



what we require is a greater contrast betwen the temperature 

 of the tropics and that of our latitudes ; so that, paradoxical 

 as it may appear, the primary cause of the "glacial" epoch 

 may be, after all, an elevation of temperature in the tropics, 

 causing a greater amount of evaporation in the equatorial 

 regions, and consequently a greater supply of the raw material 

 of snow in the temperate regions during the winter months. 



Thirdly. An alteration in the earth's axis. 



The possibility of such a change has been denied by many 

 astronomers. My father, the late Sir J. W. Lubbock, has, on 

 the contrary, maintained* that this would necessarily follow 

 from upheavals and depressions of the earth's surface if only 

 they were of sufficient magnitude. The same view has re- 

 cently been taken by other mathematicians, and among geo- 

 logists by Dr. Duncan and M. Garret. Mr. Evans has made 

 the ingenious suggestion that the solid external crust of the 

 earth may have slid over its fluid or semi-fluid nucleus. On 

 the other hand, Mr. George Darwin, who has recently dealt 

 with the subject, ( concludes that, while theoretically such a 

 change may have taken place, the amount could not have 

 been sufficient to cause any considerable change of climate in 

 a recent geological period. The subject is one of extreme 

 difficulty ; but it is at any rate clear that this suggestion, like 

 the preceding, presupposes immense geographical changes, 

 which would therefore necessarily imply an enormous lapse 

 of time. 



Fourthly. Mr. Hopkins inclines to find the true solution of 

 the difficulty in the supposition that the Gulf-Stream did not 

 at this period warm the shores of Europe. " A depression of 

 2000 feet would," he says, "convert the Mississippi into a 

 great arm of the sea, of which the present Gulf of Mexico 

 would form the southern extremity, and which would com- 

 municate at its northern extremity with the waters occupying 

 * Geol. Journ. vol. v. p. 4. f Phil. Trans, vol. clxvii. 



