l'KEHISTOKIC ANIMALS 



419 



is of the greatest value in giving us a truer conception of the 

 relationships of animals to one another. Concerning the period 

 of time necessary for the formation of the sedimentary rocks 

 we can conjecture only, for we do not know what the rate of 

 deposition in the past may have been; the time, however, must 

 in any case have been inconceivably great. 



Our oldest fossils, then, come from the Cambrian rocks. 

 Here we find remains from all the types of the animal kingdom 

 except the Chordata, but the species are all marine, with the 

 exception of some insects. These types are, furthermore, quite 

 as sharply individualized as we find them in living animals to- 

 day ; that is to say, there are no intermediate species to form 



Fin. 396. Hatterla punctata. (After Brehm, from Parker and Haswell's Manual.) 



a complete series of connecting links between them. This fact 

 does not signify that each of these types was formed during 

 that period as a group clearly differentiated from the others. 

 As we have just noted, there was life on the earth before the 

 Cambrian ; furthermore, if representatives of the Chordata had 

 existed at that period we know of no reason why they should 

 not have left fossil remains. The range of animal life, then, 

 in the Cambrian was much more limited than it is to-day, and 

 this conclusion applies not only to the types but to their various 

 subdivisions as well ; thus many classes of invertebrates were 

 not represented then at all. 



In the next period, the Silurian, we find much the same 

 groups as in the Cambrian, and in addition several new classes 

 and orders of invertebrates as well as some fishes, notably 



