ANCIENT LIFE ON THE EARTH 21 



difference between them may be due to difference in 

 geographical station. 



It appears, therefore, that out of the eight sub- 

 kingdoms into which animals are divided by zoolo- 

 gists, six were represented in the pre-Cambrian 

 times ; but, until we come close up to the Cambrian, 

 the Protozoa and Porifera alone show much diver- 

 sity ; and they were certainly the dominant feature of 

 the animal life of the early seas. No recognisable 

 vegetable remains have been found in any pre-Cam- 

 brian rock, but pelagic algae must have existed, for 

 otherwise there would have been no food for the 

 animals. 



Cambrian Life. When we pass upward into the 

 Cambrian period we find that life has made con- 

 siderable progress, including the appearance of a new 

 sub-kingdom the Echinodermata ; and in the 

 Upper Cambrian we have the first Bryozoa and 

 Pelecypoda. However, the only fossils which show 

 much variety are the Brachiopoda and the Trilobites. 



The Hydrozoa were represented by Sertularians 

 and Graptolites in the Lower, and Medusas in the 

 Upper Cambrian, the latter being so abundant that 

 the National Museum at Washington has more than 

 8000 specimens. These Cambrian Medusse belong 

 to a distinct family of the Discomedusoe called 

 Broohsellidce, and are distinguished by having a 

 lobate umbrella without any marginal tentacles. It 

 is remarkable that such soft things as jelly-fish 

 should have been preserved as fossils ; but although 

 they have no hard parts, their tissues, when saturated 

 with water, are sufficiently firm to make impressions 



