LIFE IN THE PAST 177 
esting. The swim-bladder which we have just de- 
scribed 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 animal in the egg, or, in the 
case of the mammals, before birth. It is an interest- 
ing fact that when the lung begins to form in the 
embryo it starts as a simple sac which is an offspring 
from the gullet, and occupies the position of the swim- 
bladder of the fish. This sac later divides into two, 
and develops into the lungs of the animal. This 
assures the zodlogist that the origin of the lungs in 
the higher animals is found in the swim bladder of 
the so-called lungfish. In this Silurian time certain 
of these lungfish were perhaps trapped in the basin in 
the marsh by the uplifting of the border. The waters 
becoming progressively shallower and more crowded, 
these fishes took to the land, their fins developing into 
awkward limbs which slowly became more perfect. 
To state the fact in this simple fashion is to make 
it seem far less probable than is really the case. The 
