EXTINCTION OF ANIMALS. 



77 



ancestors, for it is impossible not to call them by that name, 

 and the modern horse to its entirely different progenitors. 

 In short, as knowledge increases, the evidence from Palae- 

 ontology becomes more and more complete. 



In a general way, it is true that the simpler animals pre- 

 cede the more complex in history as they do in structural 

 rank, but the fact that all the great Invertebrate groups are 

 represented in the oldest distinctly stratified and fossiliferous 

 rocks the Cambrian system shows that this correspon- 

 dence is only roughly true. To account for this we must 

 remember that the whole mass of the oldest rocks, known 



FIG. 15. Gradual transitions between Paludina Ne-umayri (a) 

 and Paludina Hccrnesi (/). (From NEUMAYR.) 



as Archaean or Pre-Cambrian, have been so profoundly 

 altered that, as a rule, only masses of marble and carbona- 

 ceous material are left to indicate that forms of life existed 

 when these rocks were laid down. What these early forms 

 of life were, it seems impossible for us to find out, although 

 recent discoveries, for instance, of " annelid tracks " in rocks 

 of possible Pre-Cambrian age in N. W. Scotland, suggest 

 that patient investigation may yet do much towards the 

 solving of the problem. 



Extinction of Animals > 



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 



