KESULTS OF PAL.EONTOLOGICAL RESEARCH 21 



that in the stratum ' b ' which is formed after the return 

 we have before us the immediate descendants of 'a/ 

 Several such cases can be recognized with sufficient 

 certainty. Then by comparing ' b ' and ' a ' we arrive 

 under certain circumstances at a clear insight into 

 the mode of variation and its rapidity, etc. 



If the fauna of ' a/ or a group of the same, should 

 not, generally speaking, reappear, and is no longer 

 seen at the present day, then it is ' extinct/ How 

 and when it became so, we are so far ignorant. 



It is therefore seen how difficult it is to make clear 

 the process of evolution for a definite group. Many 

 geologists entirely despaired of the possibility of so 

 exact a definition of the ages of the formations as 

 was needful to that end. Incomplete, very incomplete 

 indeed, must our knowledge ever be. 



As has been stated, we can grant that for the great 

 geological formation system the relative longevity 

 has been ascertained with sufficient certainty. Be- 

 ginning with the youngest, we have arrived at the 

 following generally used table of the geological 

 periods : * 



Formation Groups. Formations. 



~ ,. (Alluvium 



1 Diluvium (Pleistocene) 



/Pliocene 

 ~, . J Miocene 



] Oligocene 



\Eocene 



1 For more exact indications and subdivisions vide Kayser : Lehrbuch 

 der Oeolog. Formationskunde, p. 7. 



