122 INVERTEBRATE PALAEONTOLOGY 



descendants. The rarity of fossils in Cambrian rocks 

 may well be due to the inadequacy of contemporary 

 skeletal tissue for preservation it cannot be ascribed 

 wholly to unsatisfactory lithological characters. It is 

 therefore reasonable to suppose that the partial failure 

 of palaeontological evidence in the Cambrian period 

 gives (inverse) premonition of its total absence from 

 earlier records. Not until the opening of the Palaeozoic 

 era had evolution produced organisms capable of 

 secreting "hard-parts." Just as in ontogeny, early 

 developmental stages are characterized by softness or 

 delicate membranous coverings; so in phylogeny, the 

 ancestral forms of a stock might be expected to lack 

 structures capable of normal preservation. 



The only Invertebrate phylum at present undiscovered 

 in Cambrian strata is that of the Polyzoa. In all of 

 the others, the types discovered are undoubtedly 

 primitive in structure, although no indication of forms 

 transitional between the phyla appears. The boundaries 

 between orders are often obscure, but the essential 

 characteristics of the great divisions had already been 

 differentiated. The palaeontological record fails just 

 where the most important events in evolution would be 

 expected to appear. 



It has often been argued that the occurrence of such 

 diversity in the fauna of the Cambrian period requires 

 invocation of a proportionately long sequence of previous 

 faunas in pre-Cambrian times. But the methods of evolu- 

 tion that can be traced in later periods, and in smaller 

 groups, must surely have operated consistently. It has 

 been shown that the most striking characteristic of neanic 

 phases of evolution is the rapidity with which important 

 innovations (often of ordinal quality) are attained. 

 Hence, the elaboration of the first Metazoan would 

 probably be followed, in a relatively short time, by 



