So 



PAST HISTORY OF ANIMALS, 



knowledge increases, the evidence from Palaeontology 

 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 correspond- 

 ence is only roughly true. To account for this, we must 

 remember that the whole mass of the oldest rocks, known 

 as Archaean or Pre-Cambrian, have been so profoundly 

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

 bonaceous 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 im- 

 possible for us 

 to find out, al- 

 though 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 

 of the problem. 



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 



FIG. 35. Gradual transitions between Paludina 

 in-// mayri (a] and Paludina /urrnesi (/). 

 From Neumavr. 



solving 



