RELATIVE DATING 23 



Organic evolution 



It is here that organic, faunal and floral, evolution comes in. 

 Sediments laid down at the same time in the geological history may 

 contain fossilized remnants of the contemporary fauna and flora. 

 Because most groups of higher organisms show evolution over the 

 geological past, there were at any time definite forms which lived 

 only at that time. If these are sufficiently distinct from both their 

 ancestors and their descendants, so as to enable us to recognize them 

 specifically from their fossilized parts, organic evolution kindly sup- 

 plied geologists with what are called index fossils. By comparing such 

 fossils, comparative ages for widely separated rock sections can be 

 established. The fact that most often 'fauna evolution' is used in this 

 context, when indeed 'organic evolution' both of the fauna and of 

 the flora is meant, stems from the circumstance that a far greater 

 number of such index fossils come from extinct faunae than from 

 extinct florae. This is mainly dictated by the very nature of the 

 fossilization process, which favours strongly the preservation of recog- 

 nizable parts of animals over plants. 



The basic idea of using organic evolution to date the geologic past, 

 be it in a relative way, is thus delightfully simple. Why then, anyone 

 familiar with geologists or geology will ask, is there divergence of 

 opinion among geologists about almost any long-distance age com- 

 parison? The reason is that, although the basic idea is simple, its 

 implementation is crowded with difficulties. Not only have similar 

 lines of development been repeated in many groups of organisms 

 throughout geologic time, so that some groups of organisms may 

 show remarkable resemblances to forms which were not directly 

 related and that lived much earlier or later, but also, most fossils do 

 not occur in all sediments formed in one given time interv^al, because 

 liv'ing things tend to concentrate in areas which are suitable for 

 them. Moreover, many groups with a relatively rapid organic evolu- 

 tion, which, of course, supply the best material for later index fossils, 

 do not occur in large numbers of individuals, and consequently their 

 fossilized remains are rare. 



Eras of relative age in geology 



However, such details do not affect us in our understanding of 

 how geology dates the past. We may safely leave the details of the 



