442 Climate of the Glacial Period. (October, 
On the continent of Europe, and in North and South 
America, no evidence whatever has been found to indicate a 
sub-tropical climate having prevailed in post-glacial times in 
the temperate regions of the globe, and I cannot but consider 
that the issue that Mr. Croll has based on the existence of 
warm climates having existed about the same time as, and 
intercalated between, his cold climates, must be given 
against him. Ifso, it is fatal to his theory, for he has not 
one whit exaggerated the importance of the necessity of these 
oscillations of temperature. If the theory be true, each 
hemisphere endured the extremes of heat and cold. Just as 
much as the Glacial period lowered the temperature of any 
place below what it is now, so must the warm period that 
came on in about ten thousand years have raised it, and it is 
a rigorous deduction from the theory that, either in the 
southern or the northern hemisphere, or both, there must 
have intervened a great period of warmth as great 
as that of the Miocene epoch since the countries were 
glaciated. 
There are some other fa¢ts to be accounted for that are 
not, I think, explained by Mr. Croll’s theory, but they will 
be better understood if I take them into consideration under 
the next theory to be discussed. 
3. Theory of a Change in the Obliquity of the Ecliptic.—So 
long ago as 1688, Dr. Robert Hooke drew attention to the 
evidences of tropical climates having prevailed in Europe, 
and speculated on changes in the axis of the earth’s 
rotation, or a shifting of the earth’s centre of gravity, or a 
change in the obliquity of the ecliptic. The last theory was 
a favourite one amongst the older English geologists, but 
even in these early days received little favour from as- 
tronomers, for Newton pronounced against it and declared 
that astronomy did not countenance the theory that there 
had been any change in the direction of the earth’s axis. 
The celebrated Laplace investigated the problem of the 
effect of the attraction of the sun, the moon, and the 
planets upon the equatorial protuberance, and came to the 
conclusion that this could only cause a variation in the ob- 
liquity to the amount of 1° 21’. More recently, Leverrier 
has examined the same question, and has arrived at the 
result that it might vary to the amount of 4° 52’, but not 
more. ‘The difference between Laplace and Leverrier is a 
large one, but most geologists have accepted their verdict as 
decisive, that former great changes of climate could not 
have been caused by variations in the obliquity of the 
ecliptic. 
