198 
ISLAND LIFE. 
[part I. 
condition of tlie northern hemisphere is the result of the 
peculiar 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 expansion at the close of the Tertiary period. 
The progress from the first was uniform and systematic : the 
