1 82 THE LIVING WORLD. 



hausted itself. It rapidly approached its limits even 

 in the Silurian (2), and then began to dwindle away, 

 and has disappeared entirely. The brachiopods had 

 also at this time reached their point of highest 

 specialization, and had become a highly developed 

 group even at this early age. Since then they have 

 remained stationary as to their organization, having 

 steadily decreased in numbers, and the few that are 

 left show no advance over the Silurian forms. The 

 cephalopod mollusks gradually increased in com- 

 plexity during the Paleozoic (2-4) ; and finally a 

 limit of the shelled forms was reached in the ammon- 

 ites of the Jurassic (7) and Cretaceous (8). The 

 culmination was followed by extinction. Meantime 

 a second line of development began, that of the 

 naked cephalopods (squids, cuttle fishes), and this 

 has gone on advancing until the present time. The 

 decapod Crustacea represent a group which is even 

 now near its culmination. From their first appear- 

 ance in the Carboniferous (4) there has been a tend- 

 ency to a concentration of organs toward the head. 

 As this specialization advanced, the abdomen be- 

 came smaller, while the head region became larger. 

 Finally, in the crabs, everything was concentrated 

 in the head region. The abdomen remained as little 

 more than rudiment. Evidently we are here near a 

 limit, and we may look upon the crabs of to-day as 

 the culmination of the special line of development 

 which has characterized this line of animals. The 

 vertebrates in general have been continually advanc- 

 ing during geological times, with a continued increase 

 in specialization and in multitude of types. But even 



