298 REPORT OF NATIONAL MUSEUM, 1892. 



the septal features of the chambered ceplialopods * is only intended to 

 show that while they really have a good degree of chronological value 

 they can not be relied upon to indicate consecutive portions of time, nor 

 as absolute tokens of substages of the geological scale. 



(7) The progress of" secular extinction of species and other divisions of the animal 

 and vegetable kingdoms, including the types which specially characterize the various 

 stages and substages of the geological scale, was accelerated hy adverse changes of 

 environing conditions and were retarded by a continuance of congenial conditions. 

 The final consummation of the extinction of the types was naturally often, and 

 perhaps usually, caused by catastrophic changes of conditions which occurred within 

 the limited areas to which they were reduced by approaching secular extinction. 



Secular extinction of all of these forms would naturally begin in those 

 localities which first became uncongenial and would be longest deferred 

 where congenial conditions lingered longest. It has thus happened 

 that certain of the types which specially characterized a given stage or 

 substage of the geological scale have survived in some parts of the 

 world long after they became extinct in other parts, and indeed after 

 the close of the stage or substage which they and their associates had 

 specially characterized. In such cases the surviving types are found 

 associated with those which characterize a later stage or substage than 

 the one in which they originated. 



In this way, for example, it is possible that a given assemblage of 

 strata in one region which upon ex parte paleontological evidence would 

 be assigned to the Cretaceous was actually c< nitemporaneous with another 

 assemblage elsewhere, which, upon other ex parte evidence would be 

 assigned to the Tertiary. A similar statement may be made concerning 

 any of the other systems or stages of the geological scale with reference 

 to those which adjoin them respectively. 



It is not necessary to infer that the locally catastrophic changes 

 which completed the final act of extinction of species and types were 

 always, or even generally, due to violent physical movements of the 

 earth's crust. Such physical changes as would diminish food supply, 

 increase the number and relative strength of enemies, alter climatic 

 conditions, or affect the quality of habitable waters were doubtless the 

 usual immediate causes of final extinction. 



(8) The geographical distribution of species within the time limits of the stages 

 and substages of the geological scale, and consequently that of the distinguishing 

 types which the species constitute, has been effected by natural means. Such means 

 included not only locomotory and mechanical dispersion within those time-limits 

 from one original center which was then the terminus of an evolutional line, but, at 

 least in the same cases, survival in various regions by separate evolutional lines 

 from the faunas of preceding stages and substages was also included. 



Because these septal features were extraneous to the bodily structure and bore no 

 known relation to that structure or to any animal function it may well be questioned 

 whether a classification based upon them is accordant with that which their anato- 

 mical structure might have furnished. Indeed the philosophical naturalist can not 

 be entirely satisfied with such a classification. The case is different, however, as 

 regards mammalian dentition, which has direct relation to essential structure and 

 bodily functions. 



