GENERAL LAWS OF ORIGIN AND MIGRATION. 253 



hemisphere as far as the Tropic of Cancer. The result is 

 that all the great equatorial currents of the ocean are im- 

 pelled into the northern hemisphere, which thus, in con- 

 sequence of the immense accumulation of warm water, 

 has its temperature raised, so that ice and snow must to a 

 great extent disappear from the arctic regions. In the 

 prevalence of the converse conditions, the arctic zone be- 

 comes clad in ice, and the southern has its temperature 

 raised. 



At the same time, according to Croll's calculations, 

 the accumulation of ice on either pole would tend, by 

 shifting the earth's centre of gravity, to raise the level of 

 the ocean and submerge the land on the colder hemisphere. 

 Thus a submergence of land would coincide with a cold 

 condition, and emergence with increasing warmth. Facts 

 already referred to, however, show that this has not al- 

 ways been the case, but that in many cases submergence 

 was accompanied with the influx of warm equatorial 

 waters and a raised temperature, this apparently depend- 

 ing on the question of local distribution of land and 

 water ; and this in its turn being regulated not always by 

 mere shifting of the centre of gravity, but by foldings occa- 

 sioned by contraction, by equatorial subsidences resulting 

 from the retardation of the earth's rotation, and by the ex- 

 cess of material abstracted by ice and frost from the arctic 

 regions, and drifted southward along the lines of arctic 

 currents. This drifting must in all geological times have 

 greatly exceeded, as it certainly does at present, the de- 

 nudation caused by atmospheric action at the equator, 

 and must have tended to increase the disposition to equa- 

 torial collapse occasioned by retardation of rotation.* 



While such considerations as those above referred to 



* Croll, in " Climate and Time," and in a note read before the British 

 Association in 1876, takes an opposite view; but this is clearly contrary 

 to the facts of sedimentation, which show a steady movement of debris 

 toward the south and southwest. 



