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 Amcebee ; 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 Coelentera, 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. 
