80 PAST HISTORY OF ANIMALS 
represented in the oldest distinctly stratified and fossiliferous 
rocks—the Cambrian system—shows that this correspond- 
ence is only roughly true. To account for this, we must 
remember that almost the whole mass of the oldest rocks, 
known as Archean or Pre-Cambrian, has been so pro- 
foundly altered, that, as a rule, only masses of marble and 
‘carbonaceous material are left to indicate that forms of life 
-existed when these rocks were laid down. Careful searching 
in Pre-Cambrian beds has revealed the presence of several 
Molluscs, a Eurypterid, and a fragment of Trilobite. There 
are also “annelid tracks” indicative of life. 
Fic. 34.—Gradual transitions between Paludina 
neumayrzt (a), the oldest form, and Paludina 
hernest (j).—From Neumayr. 
Extinction of types.—Some animals, such as some of 
the lamp-shells or Brachiopods, have persisted from almost 
the oldest ages till now, and most fossilised animals have 
modern representatives which we believe to be their actual 
‘descendants. That a species should disappear need not 
surprise us, if we believe in the “transformation” of one 
species into another. The disappearance is more apparent 
than real: the species lives on in its modified descendants, 
“different species” though they be. 
But, on the other hand, there are not a few fossil animals 
which have become wholly extinct, having apparently left 
no direct descendants. Such are the Graptolites, the 
ancient Trilobites, their allies the Eurypterids, two classes 
