2U 
HAROLD’S DISCUSSIONS. 
more aquatic life, and gradually became adapted to it. 
But its lung structure is still so far retained that it is 
obliged to come to the surface to breathe, and it is 
possible that it may yet. in future ages, become a 
thorough gill-breathing fish. 
There are fish-like creatures of several species still 
found in Australia and in some of the rivers of Africa. 
They first appeared in the Triassic period. They 
have an eel-shaped body, and breathe by gills when 
in water. But the rivers in which they exist become 
dry during several months of the year. Then they 
lie in the mud and use the air-bladder as a lung. 
In the United States lives another creature, the 
‘‘ mud-puppy,” resembling a salamander, which has 
gills and also primitive lung spiracles for air-breath¬ 
ing. A third species is the amphiuma, which diflPers 
from the ‘‘ mud-puppy ” chiefiy in that it has more 
perfect lungs and no gills, but still retains the gill 
sacs. These three species seem to be links in a chain 
of lung evolution. If this is a fact, the lungs are the 
product of environment—that is, of an attempt of the 
animal to adapt itself to the result of new conditions. 
Environment has much to do with the distribution 
of animals. There are three reasons why particular 
species are not found in certain districts: (1) High 
mountains and large bodies of water are barriers that 
can not be crossed. For instance, the civet-cat is not 
found in this country, for the reason that it has never 
been able to migrate hither. (2) Others have come 
here or been brought by man ; but they are not found 
here, because they were not able to maintain them- 
