^^^^ ZOOGENESIS "^""^ 



The significance of this imposing list of Ordovician 

 and pre-Ordovician animals becomes more evident if 

 we contemplate the missing animal types. These 

 missing types are the ctenophores or sea-walnuts (fig. 

 66, p. Ill), the flat-worms (figs. 54, 55, p. 97) and 

 round-worms (fig. 81, p. 161), the rotifers (fig. 136, 

 p. 103) and gastrotrichas (fig. 134, p. 2.03), priapulids 

 and sipunculids, heteropods, archiannelid, oligochaete, 

 myzostomid (fig. 84, p. 161), hirudinid (leeches) and 

 onychophorid (Feripatus) worms, nemerteans, pho- 

 ronids, cephalodiscids (figs. 61, 63, p. m), balanoglos- 

 sids, tunicates, and vertebrates except for fishes. 



Except for the missing vertebrates — amphibians, 

 reptiles, birds and mammals — which are primarily 

 land living, all of these various types are soft bodied 

 creatures which would be preserved as fossils only 

 under the most extraordinary circumstances. Recog- 

 nizable traces of them would only be present in the 

 rocks by the merest accident. 



While from what has just been said it is evident 

 that there has been no change whatever in the inter- 

 relationships between the phyla or major groups from 

 the very earliest times of which we have an adequate 

 fossil record up to the present day, this is not of itself 

 conclusive evidence that these groups have always 

 borne the same relation to each other. For it is 

 undoubtedly true that the Cambrian period is nearer 

 to the present epoch than it is to that far distant 

 time when life on earth began. 



Indeed, fragmentary and in some cases fairly satis- 

 factory fossils have been found in rocks of much earlier 



