550.1 327 



560 



IV 



The Imperfection of the Geological Record 



TT is now many years since Darwin first directed the special 

 -L attention of biologists to the imperfection of the geological 

 record. It was he who first satisfactorily marshalled the facts which 

 prove that the discoverable fossils in the rocks can only give a 

 very limited idea of the plants and animals which have tenanted 

 the globe at different periods in its past history. He pointed out 

 how small a portion of the earth had been geologically explored, 

 and how small a percentage of known types of life had sufficient 

 hard parts to be preserved in a fossilised state. He emphasised the 

 fact that the number both of specimens and of species preserved in 

 our museums, is absolutely as nothing compared with the number of 

 generations which must have passed away even during a single 

 geological formation. He also observed " that, owing to subsidence 

 being almost necessary for the accumulation of deposits rich in 

 fossil species of many kinds, and thick enough to outlast future 

 degradation, great intervals of time must have elapsed between 

 most of our successive formations ; that there has probably been 

 more extinction during the periods of subsidence, and more variation 

 during the periods of elevation, and during the latter the record 

 will have been least perfectly kept ; that each single formation has 

 not been continuously deposited " ; that, indeed, in every area of the 

 earth's surface there are incalculable periods of geological time 

 unrepresented in the records of the rocks. 



We may, in fact, without exaggeration declare that every item of 

 knowledge we possess concerning extinct plants and animals depends 

 upon a chapter of accidents. Firstly, the organism must find its 

 way into water where sediment is being deposited and there escape 

 all the dangers of being eaten ; or it must be accidentally entombed 

 in blown sand or a volcanic accumulation on land. Secondly, this 

 sediment, if it eventually happens to enter into the composition of 

 a land area, must escape the all-prevalent denudation (or destruc- 

 tion and removal by atmospheric and aqueous agencies) continually 

 in progress. Thirdly, the skeleton of the buried organism must 

 resist the solvent action of any waters which may percolate through 

 the rock. Lastly, man must accidentally excavate at the precise 

 spot where entombment took place, and someone must be at hand, 



