198 



ISLAND LIFE. 



[part I. 



condition of the northern hemisphere is the result of the 

 pecuhar distribution of land and water upon the globe ; and 

 the general permanence of the position of the continental 

 and oceanic areas — which we have shown to be proved 

 by so many distinct lines of evidence — is also implied by 

 the general stability of climate throughout long geological 

 periods. The land surface of our earth appears to have always 

 consisted of three great masses in the north temperate zone, 

 narrowing southward, and terminating in three compara- 

 tively narrow extremities represented by Southern America, 

 South Africa and Australia. Towards the north these masses 

 have approached each other, and have sometimes become 

 united ; leaving beyond them a considerable area of open polar 

 sea. Towards the south they have never been much further 

 prolonged than at present, but far beyond their extremities 

 an extensive mass of land has occupied the south polar 

 area. 



This arrangement is such as would cause the northern hemi- 

 sphere to be always (as it is now) warmer than the southern, 

 and this would lead to the preponderance of northward winds 

 and ocean currents, and would bring about the concentration of 

 the latter in three great streams carrying warmth to the north- 

 polar regions. These streams would, as Dr. Croll has so well 

 shown, be greatly increased in power by the glaciation of the 

 south polar land ; and whenever any considerable portion of this 

 land was elevated, such a condition of glaciation would certainly 

 be brought about, and would be heightened whenever a high 

 degree of excentricity prevailed. 



It appears to be the general opinion of geologists that the 

 great continents have undergone a process of development from 

 earlier to later times. Professor Dana says : " The North 

 American continent, which since early time had been gradually 

 expanding in each direction from the northern Azoic, eastward, 

 westward, and southward, and which, after the Palaeozoic, was 

 finished in its rocky foundation, excepting on the borders of the 

 Atlantic and Pacific and the area of the Rocky Mountains, had 

 reached its full expa.nsion at the close of the Tertiary period. 

 The progress from the first was uniform and systematic : the 



