8o 



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 Archaean 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. 



FIG. 34. Gradual transitions between Paludina 

 neumayri (a), the oldest form, and Paludina 

 harnesi (/). 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 



