96 EVOLUTION 



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 

 condition and expression of the gradual in- 

 crease of intelligent behaviour. Life becomes 

 richer and freer. (4) Increased individuation 

 makes reproductive economy possible. Pa- 

 rental care increases, and the number of off- 

 spring decreases. There is an emergence of 

 the finer feelings, and fondness is sublimed 

 in love. (5) There has been an interesting 

 peopling of the earth, an establishment of 

 faunas distinctive of the shore, the open sea, 

 the deep sea, the fresh waters, and the air. 

 Amphibians mark the important transition 

 from water to dry land; the ancient Ptero- 



