60 EVOLUTION 



or a hundred feet. The more graceful and reed-like 

 Calamites (giant horse-tails) grew in thickets from the 

 surface of the abounding lagoons. The conifers were 

 creeping slowly down to the plains, as the land rose and 

 the flash of a glacier coming from some hill here and 

 there relieved the dank and stifling monotony of the 

 swamps, and drawing nearer to the modern type. 



We will not linger over the much-debated question of 

 the relationship of these early conifers (or Gymnos- 

 perms) to the ferns and mosses (Crytogams) the 

 modern botanist sees some trace of a transition in 

 the actual Ginkgo but a word may be said on the 

 formation of the coal. The fossilised remains of the Car- 

 boniferous forests occur in seams that are often separated 

 from each other by layers of sand and mud. This led 

 earlier geologists to conceive that the land rose above 

 the water and sank again time after time, so that a 

 forest was buried under the sand and mud of the sea or 

 lake, and another forest grew above when the crust of 

 the earth arose again above the surface. Most modern 

 geologists are reluctant to admit this repeated oscillation 

 of the crust within one period. They are more disposed 

 to think that the coal-trees did not grow at the spots 

 where we find them to-day, but were washed down by 

 violent rivers from the higher ground into the lakes or 

 estuaries. Some, however, like Professor Chamberlin, 

 still hold the older view that our masses of carbonised 

 vegetation grew where we find them. 



It will be remembered, from the last chapter, that 

 after the Carboniferous period the mountains began to 

 rise, and the dry, firm land to gain on the ocean. The 

 atmosphere, too, began to grow clearer and drier, and 

 the light of the sun to penetrate more freely. As these 

 physical movements go on we find in the geological 

 record a corresponding change in the plant population. 



