204 GEOLOGICAL BIOLOGY. 



another era, and leaves us in no doubt as to the general order 

 of succession of the various genera. 



2. Although it is not improbable that in almost every 

 case the genera and the families will be found to have been 

 initiated somewhat earlier than they are now reported, and 

 new families and new genera will undoubtedly be discovered, 

 nevertheless, the outlines of the differentiation curves are so 

 emphatic in most cases that we have no reason to doubt 

 that we already have the fundamental outlines of the history 

 of each particular group of organisms clearly before us. 



3. We have here unmistakable evidence that every genus 

 and family had a definite time of initiation, and that this time 

 of initiation for each has definite relationship to the time of 

 initiation of other genera and families. 



4. Another conclusion may be drawn from an inspection 

 of the curves: the family differentiation for each grouping of 

 higher rank, suborder or order, had its evident initiation, 

 culmination, and decrease; also the generic differentiation for 

 each family had its point of initiation, its period of rapid 

 activity and culmination, and its period of decline; and in 

 many cases the actual cessation not only of expansion, but 

 of all appearance of the genus, is expressed. 



