176 THE MEANING OF EVOLUTION 
the climate of any part of the Palzozoic era. Cer- 
tainly 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 am- 
phibians 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 inter- 
