A VIE IV IN PERSPECTIVE. l6l 



constantly moist. They are therefore aquatic, or 

 occasionally live in moist earth. The Articulataand 

 Vertebrata alone are well adapted to a terrestrial 

 life, and of these immense groups only five classes 

 have really become terrestrial animals (Insecta, 

 Arachnida, Reptila, Aves, Mammalia). Five classes, 

 then, out of the whole animal kingdom are all 

 that have acquired the power of living in the air. 

 These classes have more to contend with than the 

 aquatic animals, since they have gravity to resist, 

 and must accommodate themselves to the climate ; 

 they must adapt themselves to varying temperature, 

 and to many other conditions to which aquatic ani- 

 mals are not subjected. Now it is a law of nature 

 that the being which surmounts the greatest obsta- 

 cles is the one to rise to the highest plane. It is no 

 wonder, then, that these five classes of animals have 

 become predominant types, and surpass the other 

 animals in variety and numbers. They have had 

 the whole land in their possession. 



We cannot definitely say when the terrestrial fauna 

 appeared. In the Silurian (2) there were certainly 

 in existence some scorpions, and probably, therefore, 

 'other land animals. Indeed, traces of insects have 

 been found in these rocks. Terrestrial vertebrates 

 did not appear until the end of the Carboniferous (4), 

 and not in any very great abundance until the Meso- 

 zoic (5-8). From the beginning of the Mesozoic, 

 however, terrestrial fauna has played the most impor- 

 tant part in the history of the world, and in the more 

 recent times it has been the terrestrial forms of life to 

 which evolution has been chiefly confined. 



