THE DEVONIAN PERIOD 467 



for certain conditions, being protected from their enemies by heavy 

 armor, but when the environment and food changed their very weight 

 and size were of disadvantage (p. 550), and they failed to survive. 



Why the Vertebrate Type was " Fit." — The reason for the estab- 

 lishment of the vertebrate type of animals and their rapid rise when 

 once they appeared is evident when their structure is considered. An 

 internal skeleton offers an excellent attachment for muscles, and at 

 the same time permits a great flexibility of the body. As flexibility 

 is necessary for rapid movement through the water, such animals 

 as possessed it were better able both to escape their enemies and to 

 secure their prey. Moreover, it permitted of greater size than as a 

 whole appears to have been possible in other classes. The position 

 and arrangement of the nervous system appears also to have been 

 especially advantageous. 



Plants 



In the Devonian, for the first time in the history of the world, land 

 plants are known to have been abundant. Discoveries of Silurian 

 ferns and club mosses have been announced, but they are still open 

 to doubt. The plants of the Devonian were of about the same general 

 level of organization as some of those of the present day, although 

 very different in appearance ; changes have occurred, as will be pointed 

 out from time to time, but the plants of this early period were, never- 

 theless, so highly developed as to prove an enormous antiquity. 

 " There are probably no biologists now living who oppose the doctrine 

 of evolution in toto, but if there were, they might draw a telling, 

 though fallacious argument from the high organization of the Devo- 

 nian flora." 1 (Scott.) 



At this time horsetails, ferns, club mosses, gymnosperms (of which 

 the cypress, yew, and pines are members), and the extinct spheno- 

 phylls and seed ferns (pteridosperms) are known to have existed. 

 The discussion of the Devonian flora will be taken up in the descrip- 

 tion of the Carboniferous (p. 491), since in this later period it reached 

 the climax of its development. 



Summary 



Migration and Evolution. — The faunas of the various stages of 

 the Devonian often differ so widely from one another as to suggest 



1 By the term Jlora is meant all the plants that grow in a given region or belong to a given 

 period. 



