y laden with carbon dioxide, and when
a great mass of clouds practically enveloped the earth. In this way
only do most geologists account for the enormous wealth of vegetation
in the Carboniferous period and for the abundance of plants up to the
Arctic Ocean, of the kinds that now grow chiefly in the tropics. But
of recent years a few geologists point to the fact that the peat bogs
of to-day, which seem to be the beginnings of future coal deposits,
are found almost entirely in cold countries. Hence it is a serious
matter to attempt to describe the climate of any part of the
Palaeozoic era. Certainly of the climate earlier than the Carboniferous
it is very risky to say anything definite.

The forests of the coal period seem actually to have cleared the air;
at least now we begin to find creatures related to our salamanders and
frogs moving about among the stumps of the marshes. These amphibians
are evidently the descendants of some of the fishes of the Devonian
times. Among these fishes were some which bear a great resemblance to
a few found in South America, in Africa and Australia to-day, and
which we know as lungfish. Anyone who has cleaned our fresh water
fishes in preparation for the table will remember that inside of them
there is a long slender bladder filled with air. This bladder assists
in making the fish light, hence making it easier for it to support
itself in the water. In certain swampy regions these lungfish swim
freely in the water of the marshes. When the dry season comes,
however, the water evaporates, draining the marshes completely. This
would prove the death of most fishes. The lungfish have a curious
habit which keeps them over the dry season. They cover themselves with
a coat of mud, inside of which there is a lining of slime produced
from their bodies. In such cocoon-like cases they survive the drought.
The means by which they breathe during this dry season is
interesting. The swim-bladder which we have just described in other
fishes is, with this lungfish, peculiarly spongy in its walls,
presenting a large surface full of blood vessels which absorb the air
on the inside of the bladder. This air the fish changes with moderate
frequency, the result being that the swim-bladder serves him exactly
as the lung serves a higher animal. To this fact he owes his name of
lungfish.

We sometimes gain much light concerning the past history of any
particular form of animal by studying the development of that