A VIEW IN PERSPECTIVE. 12$ 



increased rapidly in numbers and diversity, soon 

 reached a culmination, and then disappeared with 

 an almost equal rapidity (see Fig. 15). The tri- 

 lobites, for instance, were numerous in the Silurian 

 (2). During this age they became highly diversified, 

 reached their culmination in the last of the same 

 age, and then rapidly diminished in numbers and 

 disappeared. The same is true of euryptids. The 

 graptolites were abundant in the Lower Silurian (2), 

 reached their culmination during this age, and dis- 

 appeared completely with it, no traces of them being 

 found in any subsequent period. Other examples 

 of the same kind are the Cystiphilloidea, disappear- 

 ing in the Devonian (3), the Blastoidea and Cistoidea 

 in the Carboniferous (4), and also the Palaeechinoidea 

 ending their history in the Carboniferous (4). 



Fig. 1 6. Some groups not in existence at the be- 

 ginning of the history seem to have suddenly appeared 

 and then as suddenly disappeared again (see Fig. 16). 

 The rugosites of the Cretaceous (8) are the best ex- 

 amples of this. These remarkable mollusks, so unlike 

 any other representatives of the group, seem to have 

 become quite abundant in that age. They completed 

 their history, however, with the Cretaceous, and no 

 traces of them subsequently have been found. None 

 other of the invertebrate orders had a similarly short 

 life. One order of the dipnoids (Acanthodini), one 

 of Amphibia (Stegecephala), six orders of reptiles 

 (Ichthosauria, Plesiosauria, Pythonomorpha, Thero- 

 morpha, Dinosauria, and Pterosauria) have been 

 confined to comparatively short periods in the past. 

 Three orders of birds (Saururae, Odontholcae, Odon- 

 totormae) and at least five orders of mammals 



