76 INVERTEBRATE PALAEONTOLOGY 



their highest development successively. The time dur- 

 ing which such a lineage maintains its acmaic qualities, 

 biological and numerical, is called the " hemera " of 

 that series. In strict terminology, any deposits that 

 may accumulate during the hemera are the "zone" 

 characterized by the group of organisms. If this dis- 

 tinction is borne in mind, it becomes manifest that zones 

 may, or may not, be represented in various places, their 

 existence depending on opportunities for rock-deposition 

 during their hemerae. A hemera is a section of time 

 (as the name suggests) ; while a zone is the tangible 

 result of local episodes transacted in the specified 

 interval. Thus, while it is perfectly justifiable to record 

 that a zone does not occur in a certain locality, it would 

 be absurd to suggest that a hemera was absent, although 

 it may be unrepresented by actual materials. 



The least satisfactory character of hemerae is their 

 determination through evidence supplied by particular 

 fossils. Although most species prove to have wide 

 ranges at their times of acme, they rarely become 

 world-wide the most cosmopolitan forms are usually 

 those of long duration whose extended acmes show little 

 prominence. But a race of organisms whose vigour is 

 such that its members would be chosen as hemeral 

 indices will normally have representatives in all parts 

 of the world where conditions are suitable ; and the 

 marked tendency to synchronous parallelism in the 

 evolution of homogenetic stocks will usually produce 

 " complementary " species that replace the missing guides 

 and fulfil their r61e. In consequence, it is often most 

 satisfactory to consider a hemera to be indicated by a 

 rnorphogenetic " mode/' or phase of specialization, than 

 by any special specific types. 



A zone may be recognized by the presence of the 

 hemeral index in an acmaic stage of evolution, but is 



