1874.] Climate of the Glacial Period. 461 
But long nights mean extreme cold: the one cannot occur 
without the other. The earth rapidly radiates its surface- 
heat into space, and, if the loss be not compensated for by 
what is received from the sun, the temperature soon falls 
far below the freezing-point. 
Neither could any possible increase in the eccentricity of 
the earth’s orbit alter materially the length of the Arctic 
night; nor could the moderate amount of change allowed 
by astronomers in the obliquity of the ecliptic. Taking 
their highest limit, the Arctic night in lat. 78° 56’ would 
still last for three months, during which the sun would 
not rise above the horizon. It is impossible but that the 
radiation from the earth during that time would produce 
intense cold. This long night could be lessened in one 
way, and one way only,—by a much greater change in 
the obliquity of the ecliptic than astronomers have yet 
admitted can have taken place. 
In an enquiry of this kind it is well when we can get 
down to such a crucial fa¢t as is that of the flourishing 
of many species of large trees so far within the Arctic 
Circle. It is of far more importance than any or all the 
arguments I have used about the Glacial period. It admits 
of but one explanation. The long Arctic nights are caused 
by the obliquity of the ecliptic, and only by the lessening of 
that obliquity can they be shortened. There is no reason 
to believe that this vegetation could have been fitted to 
endure extreme cold. It belongs to many different genera, 
and the greater part are not these that are now characteristic 
of cold regions. For these, according to Heer, we should 
have to go to the very Pole itself. At thesame time, in North 
Greenland, flourished a flora that could only live where frost 
was unknown. Many of the same species lived much 
further south along with subtropical forms that prove that 
the climate of Central Europe was then both much warmer 
and more equable than it now is, and Heer considers that 
the mean temperature of North Greenland would have to be 
raised at least 29° F. to enable the Early Tertiary flora to 
flourish there. 
I have shown that a great increase in the obliquity of the 
ecliptic would produce the cold of the Glacial period, let us 
now consider what would be the effect of a great decrease 
in that obliquity. Would it tend to produce conditions 
faveurable for the growth of vegetation up to the North 
Pole? It will simplify the question by investigating how 
far an entire obliteration of the obliquity would ameliorate 
the climate of the Arctic regions. The present position of 
Wor. IV. (N.S.) 3N 
