CHAPTER XII. 



THE GEOLOGICAL SUCCESSION OF ANIMALS. 



THE different systems of rocks, from the Silurian to the 

 Quaternary or present age, contain the fossil remains of ani- 

 mals, which show that in the beginning the animals were, 

 as a whole, unlike those now living, the later types becom- 

 ing more and more like those now constituting the earth's 

 fauna. The oldest set of animals, the Palaeozoic, comprised 

 species of nearly all the branches of: invertebrates, with a 

 few fishes. A large proportion of these animals belonged 

 either to simple or to what are called generalized types, 

 though some were as specialized as any invertebrates now 

 living. Progress upward has involved the disappearance of 

 most of the generalized types, and their replacement by more 

 or less highly specialized types. Thus the earliest corals were 

 mostly of the Rugose type, which were succeeded by the 

 more complicated recent forms ; the Brachiopods or shelled 

 worms were replaced by mollusks ; the generalized trilobites 

 gave way to the genuine specialized shrimps and crabs ; tbe 

 existing generalized king-crab, with its affinities to spiders, 

 has survived a number of still more generalized or synthetic 

 allies. The generalized sharks and ganoids abounded at a 

 time when there were no bony fishes like the cod and her- 

 ring. Nearly nine thousand species of bony fishes have 

 appeared since the extinction of the earlier types of cartila- 

 ginous and mail-clad fishes. The highly specialized horse 

 was preceded by a number of more generalized species and 

 genera, the oldest of which approached the tapir, one of the 

 most generalized of mammals. The succession of forms 

 leading up to the horse is paralleled by the succession of 



