evolution," I am ignoring or defying this view
of many distinguished geologists. I am taking careful account of it.
There is no dispute, however, about the fact that the Permian age
witnessed an immense carnage of Carboniferous organisms, and a very
considerable modification of those organisms which survived the
catastrophe, and that the great agency in this annihilation and
transformation was cold. To prevent misunderstanding, nevertheless, it
will be useful to explain the controversy about the climate of the earth
in past ages which divides modern geologists.

The root of the difference of opinion and the character of the
conflicting parties have already been indicated. It is a protest of the
"Planetesimalists" against the older, and still general, view of the
origin of the earth. As we saw, that view implies that, as the heavier
elements penetrated centreward in the condensing nebula, the gases were
left as a surrounding shell of atmosphere. It was a mixed mass of gases,
chiefly oxygen, hydrogen, nitrogen, and carbon-dioxide (popularly known
as "carbonic acid gas"). When the water-vapour settled as ocean on the
crust, the atmosphere remained a very dense mixture of oxygen, nitrogen,
and carbon-dioxide--to neglect the minor gases. This heavy proportion of
carbon-dioxide would cause the atmosphere to act as a glass-house over
the surface of the earth, as it does still to some extent. Experiment
has shown that an atmosphere containing much vapour and carbon-dioxide
lets the heat-rays pass through when they are accompanied by strong
light, but checks them when they are separated from the light. In other
words, the primitive atmosphere would allow the heat of the sun to
penetrate it, and then, as the ground absorbed the light, would retain
a large proportion of the heat. Hence the semi-tropical nature of the
primitive earth, the moisture, the dense clouds and constant rains that
are usually ascribed to it. This condition lasted until the rocks and
the forests of the Carboniferous age absorbed enormous quantities of
carbon-dioxide, cleared the atmosphere, and prepared an age of chill and
dryness such as we find in the Permian.

But the planetesimal hypothesis has no room for this enormous percentage
of carbon-dioxide in the primitive atmosphere. Hinc illoe lachrymoe: in
plain English, hence the acute quarrel about primitive climate, and
the close scanning of the geological chronicle for indications that the
earth was not