39 2 THE VERTEBRATES. 



Devonian; reaching down through them to the arthrodires and to the ostracoderms 

 of the Silurian, and then again beyond them to the merostomes, trilobites, and 

 primitive phyllopods of the Ordovician, Cambrian, and Proterozoic. 



In this vigorous phylum, evolution follows a logical and consistent course, 

 each important event being the direct, or indirect result of the preceding ones, 

 and they themselves creating the conditions that bring about those that follow. 

 The various independent sets of organs, such as the brain, sense organs, appen- 

 dages, jaws, internal and external skeleton, in their own peculiar ways move steadily 

 onward toward the same end, in a manner that could hardly be possible except 

 in a real, not an imaginary, line of evolution. The perfection with which these 

 immensely varied and complicated facts and details fit together to form a definite, 

 intelligible picture, carries with it the overwhelming conviction that that picture 

 is an image of the truth. The precision with which each event creates again and 

 yet again, new conditions, new organs, and new readjustments, shows that the 

 primary forces that sustain and direct the main lines of evolution are self-creative, 

 and lie within, not without, the organism. 



