KESULTS OF PAL^ONTOLOGICAL RESEARCH 19 



consider the mode of expression used by many 

 authors. 1 



The uncertainty which exists, if we accept the usual 

 division into separate groups determined by age, may 

 be judged by the following short consideration : If 

 it be no longer a question whether the organisms gener- 

 ally vary, but rather how they transformed themselves, 

 then it is not sufficient to compare formations differing 

 considerably in age, but those immediately following 

 each other must be known, since it is only when it is 

 known which formation was the next to be deposited, 

 that the further fate of a definite organic group can 

 be properly followed up without a break. Then next 

 younger, which we will call ' b/ need not necessarily be 

 deposited over stratum ' a ' which has just been formed, 

 but may originate in quite another region. The stratum 

 e a ' can, for instance, become dry land by the retreat of 

 the sea in which it was formed. The sea itself departs, 

 together with its organisms, which hitherto had been 

 buried in 'a/ to some other region and there deposits the 

 successors of the organisms buried in "a/ If there be 

 no means of recognizing this next younger deposit, 

 or if it be again covered by the sea, then nothing can 

 be said regarding the evolutionary progress of such a 

 group, or at least there exists a gap. Then it may 

 happen that the animal groups, which we learnt to 

 recognize in the strata complex 'a/ in that deposit (' c ') 



1 Compare the methods of age-determination by E. Kayser (Lehrbuch 

 der Oeolog. Formationskunde, Stuttgart, 1909, p. 2) ; and also the Introduc- 

 tion of M. Neumayr, Erdgeschichte II. 



c2 



