174 THE MEANING OF EVOLUTION 
fauna is to be noticed. In the Devonian the plants 
are creeping up upon the ground. Ferns are growing 
about everywhere, though they are not exactly our 
ferns, but are rather a sort of intermediate form be- 
tween these and the present seed plants. 
Now comes an entire change in the history of the 
world. By some means a rise in the bottom seems 
to have cut off a great part of the internal sea from 
the outer ocean and to have converted it into a wide- 
spread shallow bay, much like the sounds which lie 
back of the islands that line the Atlantic Coast from 
New Jersey to Florida. Just as this coastal region 
to-day is covered with salt marshes, so the whole in- 
ternal sea of the Carboniferous period was converted 
into a great swamp. Sometimes an oscillation of the 
crust of the earth brought this marsh above the sur- 
face of the sea and a luxuriant growth of plants 
spread over it. Then a sinking of the bottom allowed 
the mud and sand to wash down the shores, and spread 
out over the marsh, and enclose the muck of the 
marsh under a layer of sand or clay. Another lift 
of the bottom would start the swamp growing once 
more, and a series of alternations between marsh land 
and sound seems to have followed. The plants of 
this period are not the plants of to-day, though we 
still have some very degenerate representatives of 
them. The common horse-tail, with its angular, 
