GREAT STEPS IN EVOLUTION 97 



us notice some of the great acquisitions 

 bilateral symmetry, a head-brain, specialized 

 sense-organs, a body-cavity, a segmented 

 body, muscular feet, a renewable external 

 armour, muscular jointed appendages, and so 

 on. Or let us think of particular cases such 

 as the extraordinary development of the re- 

 spiratory system in insects, where ramifying 

 tubes carry air to every nook and cranny of 

 the body, so that the blood can hardly ever 

 become impure, and a climax of vital activity 

 is attained. The story of the evolution of 

 Invertebrates is a succession of great achieve- 

 ments. Among Vertebrates they were even 

 greater. 



THE ASCENT OF VERTEBRATES. It is a 

 profitable exercise to draw a long ascending 

 slope, with perpendiculars dropped from 

 various points to the base line, registering the 

 evolutionary ascent of Vertebrate animals. 

 What are the impressive facts ? (1) Life has 

 been creeping or pressing upwards for many 

 millions of years. (2) Great specializations 

 occur at every level, but there is also beyond 

 doubt a progressive differentiation and in- 

 tegration. (3) This is particularly true of 

 the nervous system, and is of course a con- 

 dition and expression of the gradual increase 

 of intelligent behaviour. Life becomes richer 



D 



