1874.] Climate of the Glacial Period. 447 
the snow andice piled up around the poles; and we may now 
inquire if the theory throws any light on other problems of 
the Glacial period, and is in harmony with the facts of 
geology. In doing this I shall contrast it with the other 
cosmical theory. The theory of the greater eccentricity of 
the orbit requires that the glacial periods of the two hemi- 
spheres should be at different times; that of the greater 
obliquity of the ecliptic, that they should be simultaneous. 
There is not much evidence available, but what little there is, 
is in favour of the glaciation of the two hemispheres having 
occurred at the same time. Thus, there exist glacial con- 
ditions at present around the poles, due primarily to the 
obliquity of the ecliptic, and these conditions are contempo- 
raneous in the two hemispheres. More ice and snow is 
heaped up within the antarctic circle than at its antipodes, 
because a greaterevaporating area of ocean surroundsit, whilst 
the arctic regions are almost circled by land that not only 
lessens the evaporating surface, but intercepts much of the 
moisture-bearing currents from the south. The snow piled 
up on the Himalayas, the Alps, and other high northern 
ranges, is just so much prevented from reaching the arétic 
regions. That the difference is due to lessened precipitation, 
and not to a difference of temperature, will be seen if we 
follow the isotherm of 30° around each hemisphere. We 
shall find it deviating but little in the southern hemisphere 
from the line of lat. 60°, being now a little to the north and 
now a little to the south of it. In the northern hemisphere 
the isotherm of 30° is much more irregular, sometimes 
running far to the south, sometimes far to the north, but 
the mean is again about lat. 60°, proving that if there was 
as much precipitation there would be as much ice and snow 
fo the north of lat. 60° N. as there is to the south of 
lat. 60° S. Even now, if all the snow of northern mountain 
ranges was added to that existing to the north of lat. 60° N., 
the difference would be greatly lessened, and we should 
have in both hemispheres a partial Glacial period reaching 
nearly 30° from the poles, and produced by the present obli- 
quity of the ecliptic. Only on one of the other planets has 
an accumulation of snow at the poles been proved to exist, 
namely, on Mars; which, with an obliquity of 303°, is glaci- 
ated at both poles at the same time. So that, judging from 
analogy, we might expect the glacial period of the two 
hemispheres to have been contemporaneous. 
Many plants and some animals are found, in both the 
northern and southern temperate zones, separated by the 
whole width of the tropics, which they cannot now pass; 
