CHAPTER X11, 
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 carth’s 
fauna. The oldest set of animals, the Paleozoic, 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 ; the 
existing generalized king-crab, with its affinities to spiders, 
has survived a number of stili 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 
