THE MODIFICATION OF GENERIC CHARACTERS. 293 



A Culminating Point or Acme in the Life-period of a Genus. — 

 Attain, we observe that the fossil specimens which present the 

 characters (of Meristella for instance) are most abundant 

 along the middle of this period; for the Meristellas it is about 

 the Neosilurian; also in that period they are more frequently 

 met with in distant parts of the world ; and where they are 

 most abundant the characters which serve to distinguish them 

 into separate species are more numerous ; and both before that 

 epoch and afterward there are fewer and fewer, until we reach 

 both ends, where the species are very rare. 



Summary of the Geological Characteristics of a Genus. — To 

 generalize the above observations, it may be said that the 

 genus practically has a time of beginning and a time of ending. 

 Practically, that is, according to the knowledge we possess, 

 there was a geological time, represented by a particular horizon 

 in the geological series of strata, when each genus began; 

 there was a particular period, of shorter or longer extent, 

 during which the genus was freely propagated, and abundant 

 individuals flourished, leaving their remains in the strata, 

 wherever the conditions were appropriate for their preserva- 

 tion. The genus had a period of decadence, or of growing 

 old, the species became fewer and fewer, the individuals more 

 rare, and finally the genus died out, and, so far as our knowl- 

 edge goes, became extinct. These laws apply to Meristella, 

 and in substance they apply to all genera w^e know of. The 

 period from the initiation to the extinction of the genus is the 

 life-period of that genus. 



