A VIEW IN PERSPECTIVE. 



I2 9 



been constantly increasing in range with greater 

 or less rapidity. This is true of the hydroids, 

 corals, star-fishes, mollusks, and especially of the 

 air-breathing insects. 



Fig. 19. Some groups not present at the beginning 

 of the Silurian appeared subsequently, perhaps in 

 small numbers, and from the time of their appearance 



FIG. 19. 



have continued to expand (see Fig. 19). Their num- 

 bers have grown larger and their diversity of structure 

 has constantly increased. This has continued until 

 to-day, and now they exist in greater abundance 

 than ever before. Under this head are included all 

 of the vertebrate orders living to-day, and nearly all 

 of the insects, at least of the higher orders. Among 

 the lower animals we find, it true of the dibranchiate 

 9 



