300 REPORT OF NATIONAL MUSEUM, 1802. 



Sucli a commingling of types is known to occur upon the confines of 

 other systems as well as the < 'orboniferous, and the discovery of similar 

 faunal conditions is to be expected in the case of any of them in all 

 regions where the successive series of stratified rocks is complete. 

 That is, where there is a sharply defined boundary between any two 

 systems it has been due to such physical changes as broke the conti- 

 nuity of sedimentation and of life for the region in which it occurred. 



(10) Although movements and displacements of the earth's crust have from time 

 tn time occurred over large portions of its surface, arresting sedimentation or chang- 

 ing its character and causing great destruction of life, there has never heen a uni- 

 versal catastrophe'of that hind. On the contrary, during all the time that disastrous 

 conditions prevailed in any given area, conditions congenial to the existence and 

 perpetuity of life prevailed in other and greater areas. 



Tt is this persistence of congenial physical conditions over large por- 

 tions of the earth's surface while smaller portions were disastrously 

 affected that has not only insured the perpetuity of life in general, but 

 that has insured the survival of certain chronological types of living 

 forms in some parts of the earth after their complete extinction in 

 other parts. Furthermore it is the evidence of the unbroken continuity 

 of sedimentation attending those congenial conditions, as well as that 

 of the unbroken continuity of life, which renders it difficult and often 

 impracticable to draw distinct physical, as well as biological, lines of de- 

 marcation between contiguous stages and snbstages of the geological 

 scale, especially when attempting to determine the correlation of the 

 divisions of the scale for different parts of the world. 



The second of the two sets of propositions, together with the accom- 

 panying remarks, which are recorded on the preceding pages, show 

 that certain of the views held by the early geologists, notably those 

 which assumed the universally sharp definition of all the divisions of 

 the geological scale, were radically wrong. Still, it is evident to every 

 one who is familiar with modern geological literature that those views 

 have continued to exert an adverse influence upon the biological 

 branch of geological investigation long after they have been formally 

 rejected, even by those who continued to be influenced by them. The 

 early geologists adopted methods of investigation which were consis- 

 tent with their biological views, but I have shown that from the pres- 

 ent standpoint of biology certain of those views were so fundamentally 

 wrong that the methods which were based upon them are quite out of 

 place in modern investigation. Still, those methods of our energetic 

 predecessors have come down to the present time with such force and 

 with such evidence of the general correctness of the scale which they 

 had established by them that it has been difficult for their successors 

 to adopt the modification of methods which has been necessitated by 

 the great subsequent revolution in biological thought ami methods of 

 investigation. 



The facts which have been presented on the preceding pages show- 

 that, while the scale which the early geologists established is a wonder- 



