114 THE LIVING WORLD. 



Not a few of them reached their culmination early in 

 the Silurian (2), and soon after became extinct. In 

 other words, the sub-kingdoms and classes of ani- 

 mals, and, in many cases, the orders and families, 

 were as distinct from each other in the beginning of 

 the Silurian (2) as they are to-day. The divergence 

 of types had fully taken place prior to the beginning 

 of our fossil record. Except in the one division of 

 Vertebrata, the development of classes and orders 

 must be learned from embryology and anatomy ; the 

 study of fossils is entirely inadequate to help us to 

 any connected history. 



Paleozoic, Mesozoic, and Cenozoic Epochs. 



A second equally striking fact is that in the long 

 history that has intervened between the Silurian (2) 

 / and the present time there have been two specially 

 prominent dates. The first was at the beginning of 

 the Mesozoic (5-8). During the Paleozoic, as we 

 have seen, various types of animals had existed in 

 abundance ; but, though the three Paleozoic ages 

 were of an immensely long duration, the modifica- 

 tions of type and the production of new families 

 during its progress were comparatively slight. With 

 the beginning of the Mesozoic, however, there was a 

 remarkable expansion and development of almost all 

 classes of animals. Within a comparatively short 

 time a greater expansion into new forms took place 

 than had occurred during all of the Silurian, Devon- 

 ian, and Carboniferous taken together. A glance at 

 the diagrams (Figs. 10-14) will show this expansion 

 in a striking manner. The fact that there was such 



