260 Notices of Books . [April, 
perature towards the poles was much less marked than at 
present. 
As regards the causes of those alternations of temperature 
from the Lower Miocene to the Glacial, our author refers — but 
with only partial approval — to the influence of different distribu- 
tions of land and water upon the earth’s surface. The ArCtic 
Ocean was probably, during the Glacial epoch, in connection 
with the Baltic, and the Sahara was — according to Prof. Escher 
de la Linth — submerged by the sea.* 
Prof. Heer then examines four of the hypotheses advanced for 
the explanation of the phenomena in question. These are — 
Change of climate from the diminution of the earth’s natural 
heat ; modification of the sun itself ; change of position of the 
earth with regard to the sun ; and irregularity of temperature in 
ethereal space. 
The first of these hypotheses would agree well with a con- 
stantly decreasing temperature, but it is utterly inadequate to 
explain a double period of intense cold with a milder epoch in- 
tervening, and succeeded by a more genial climate. Dr. Blandet’s 
hypothesis, of a progressive decrease in the solar heat, is equally 
inadmissible. The third hypothesis, Mr. Croll’s,t which seeks 
the cause of such alternating extremes of temperature in the 
periodically varying eccentricity of the earth’s orbit, is rejected 
by our author on the ground that the plants and animals pre- 
served in the rocks by no means confirm Mr. Croll’s theory. 
The fact that the northern half of the globe has warmer sum- 
mers than the southern atmosphere is ascribed to the different 
distribution of land and sea, the writer forgetting that the summer 
of the southern hemisphere is shorter than that of the northern, 
from the greater orbital velocity of the earth when in perihelion. 
Prof. Heer seems to lean to the fourth and last hypothesis, 
which seeks the cause of warm and cold periods in the temper- 
ature of different regions of space. “ The Miocene period may 
be compared to the summer, the Glacial period to the winter, 
and the existing geological age to the spring of the planetary 
system.” 
But it is inconceivable that the temperature of different regions 
of space can vary, save as far as is due to the greater or less 
number and propinquity of stars radiating heat. Now if our 
earth, or rather our planetary system, passed so near such stars 
as to raise the average temperature of Europe by i6° F., their 
attraction would undoubtedly have exerted a very decided in- 
fluence on the orbits of the earth, the other planets, and their 
satellites. Yet of such perturbations astronomers can find no 
traces. Pc will at once appear that, on the first, second, and 
fourth of these hypotheses, glaciation would occur in both the 
* There is grave reason to fear that the schemes for letting in the ocean 
upon the Sahara might have disastrous effe&s upon the climate of Europe. 
f Quarterly Journal of Science, vol. iv., p. 421, and vol. v., p. 3og. 
