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 conies 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, 



