7 6 



PAST HISTORY OF ANIMALS: 



though less perfectly, the modern crocodiles are linked by 

 many intermediate forms to their extinct ancestors, for it is 

 impossible not to call them by that name, and the modern 

 horse to its entirely different progenitors. In short, as know- 

 ledge 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 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. What these early forms of life were it seems impos- 

 sible 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 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 



Fig. 32. — Gradual transitions between Paludina 

 Neumayri (a) and Paludina Hosrnesi (j). — 

 From Neumayr. 



