EXTINCTION OF TYPES 



lOI 



spondence 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 

 profoundly 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 ** annehd tracks " indicative of 

 life. 



Extinction of types. — Some animals, such as some of 

 the lamp-shells or Brachiopods, have persisted from almost 



Fig. 44. — Gradual transitions between Paludma 

 neumayri (a), the oldest form, and Paludina 

 hcernesi (;'). — From Neumayr. 



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 

 of Echinoderms (Cystoids and Blastoids), many giant 

 Reptiles, and some Mammals, 



