78 PAST HISTORY OF ANIMALS. 



teeth, and shells, are likely to be preserved, and this at once 

 greatly limits the evidential value of fossils. The primitive 

 forms of life would almost certainly be without hard parts, 

 and have left no trace behind them. A number of ex- 

 tremely interesting forms, such as many worms and the 

 Ascidians, are, for the same reason, almost unrepresented 

 in the rocks. Finally, we cannot suppose that such an 

 external structure as a shell can always be an exact index of 

 the animal within. 



After fossilisation has taken place, the rock with its con- 

 tents may be entirely destroyed by subsequent denudation, 

 or so altered by metamorphic changes that all trace of 

 organic life disappears. Of those fossils which have been 

 preserved only a small percentage are available, for vast 

 areas of fossiliferous rocks are covered over by later deposits, 

 or now lie below the sea or in areas which have not yet 

 been explored. 



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. Further, those specially favourable cases which 

 have been completely worked out have yielded results which 

 strongly support the general theory. 



Probabilities of " fossils." 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 preservation, 

 and the same may be said of forms like Amoebae ; while the Foramin- 

 ifera and the Radiolaria, having hard structures of lime or silica, have 

 been well preserved. The flinty Sponges are well represented by their 

 spicules and skeletons. Of the Ccelentera, 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 vestiges of jelly-fish have been discovered. 



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

 actual remains ; the others are known only 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. 



