CHAP. XXIV.] 
SUMMARY AND CONCLUSION. 
503 
case with Northern Europe during the glacial epoch — then the 
glacial conditions would he continued and perhaps even inten- 
sified when the sun approached nearest to the earth in winter, 
instead of there being at that time, as Mr. Croll maintains, 
an almost perpetual spring. This important result is supported 
by reference to the existing differences between the climates 
of the northern and southern hemispheres, and by what is 
known to have occurred during the last glacial epoch ; and it 
is shown to be in complete harmony with the geological evidence 
as to interglacial mild periods. 
Discussing next the evidence for glacial epochs in earlier 
times, it is shown that Mr. Croll'* s views are opposed by a vast 
body of facts, and that the geological evidence leads irresistibly 
to the conclusion that during a large portion of the Secondary 
and Tertiary periods, uninterrupted warm climates prevailed in 
the north temperate zone, and so far ameliorated the climate of 
the Arctic regions as to admit of the growth of a luxuriant 
vegetation in the highest latitudes yet explored. The geogra- 
phical condition of the northern hemisphere at these periods is 
then investigated, and it is shown to have been such as to 
admit the warm tropical waters freely to penetrate the land, 
and to reach the Arctic seas by several channels ; and, adopt- 
ing Mr. Croll’s views as to the enormous quantity of heat that 
would thus be conveyed northwards, it is maintained that the 
mild Arctic climates are amply accounted for. With such 
favourable geographical conditions, it is shown, that changes of 
excentricity and of the phases of precession would have no other 
effect than to cause greater differences of temperature between 
summer and winter ; but, wherever there was a considerable 
extent of very lofty mountains the snow-line would be lowered, 
and the snow-collecting area being thus largely increased a con- 
siderable amount of local glaciation might result. Thus may be 
explained the presence of enormous ice-borne rocks in Eocene 
and Miocene times in Central Europe, while at the very same 
period all the surrounding country enjoyed a tropical or sub- 
tropical climate. 
The general conclusion is thus reached, that geographical 
conditions are the primary causes of great changes of climate, 
