IMPERFECTION OF THE GEOLOGICAL RECORD. 69 



(6) That many fossiliferous rocks have been so altered 

 in their nature that all traces of their origin and their fossils 

 have disappeared. 



With all these causes operating against the likelihood of 

 preservation, and of finding those forms that may have been 

 preserved, it is little wonder if the geological record is 

 incomplete; but such as it is, it is in general agreement with 

 what the other evidence, theoretical and actual, leads us to 

 expect as to the relative age of the great types of animal 

 life. We have reason to wonder, not at the incompleteness 

 of the geological record, but rather that the traces of past 

 history are not even more fragmentary than they are. 



Probabilities of "fossils" in the various classes of animals. — 

 But it will be useful to note the probabilities of a good 

 representation of extinct forms in the various classes of 

 animals. Thus among the Protozoa the Infusoria have no 

 very hard parts, and have therefore almost no chance of pre- 

 servation, and the same may be said of forms like amoebae ; 

 while the Radiolaria and the Foraminifera, having hard 

 structures of lime or silica, have been well preserved. The 

 Sponges are well represented by their spicules and skeletons. 

 Of the Ccelenterates, except an extinct order known as 

 Graptolites, only the various forms of coral had any parts 

 readily capable of preservation, and remains of these are 

 very abundant in the rocks of many ancient seas. But, 

 strange as it may seem, some beautiful remains of jellyfish 

 have been discovered. 



Of the great series of "worms," only the tube-makers have 

 left actual remains, the others are only known by their 

 tracks, while of any that may have lived on the land there 

 is no evidence. 



The Echinoderms, because of their hard parts, are well 

 represented in all their orders except the Holothurians, where 

 the calcareous structures characteristic of the class are at a 

 minimum. 



The Crustacea, being mostly aquatic, and in virtue of 

 their hard skin, are fossilised in great numbers. 



The Arachnida and the Insects, owing to their air-bteathing 

 habit, are chiefly represented by chance individuals that have 

 been drowned, or enclosed within tree stumps and amber. 



The MoUusca and Brachiopods are perhaps better pre- 



