PAST HISTORY OF ANIMALS. 75 



servation of any given animal are very great. Many are 

 destroyed by other living creatures, or obliterated by 

 chemical agencies. Except in rare instances, only hard parts, 

 such as bones, teeth, and shells, are likely to be preserved, 

 and this at once gready limits the evidential value of fossils. 

 The primitive forms of life would almost certainly be with- 

 out hard parts, and have left no trace behind them. A 

 number of extremely interesting forms, such as many worms 

 and the Ascidians, are, for the same reason, almost unrepre- 

 sented 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. Some shells, such as Natitilus and some 

 of the Brachiopods, occur as fossils from remote Palreozoic 

 ages onwards, but it is impossible to believe that the animal 

 within has never varied during this period, though we cannot 

 now learn either the nature or the amount of the variation. 



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 these fossils which have been preserved 

 only a small percentage are available, for vast areas of fossili- 

 ferous rocks are covered over by later deposits, or now lie 

 below the sea or in lands 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" 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 chancer of preservation, and the same may be said of forms 

 like Amoeba." ; while the Foraminifera and the Radiolaria, 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 



