190 



THE CHAIN OF LIFE. 



now extant, ot, which but one occurs in America, unless 

 P. Mexicana, the Mexican plane-tree, is a good species as 

 distinct from the ordinary, more northern, form. There are 

 seven species, according to Lesquereux, in the Cretaceous o'' 

 Dakota alone. This sort of evolution backw. rd, or from 

 many species to few, would probablv be greatly increased, 



.^v 



Fig. \i^.— Sassafras cretaceiimi^^^htxty). 



had we more full knowledge of the Cretaceous flora, as there 

 are several genera already represented by as many species as 

 they can boast in modern times. We have already seen that 

 this abrupt and sudden culmination of genera and families, and 

 their subsequent decadence, is no rare thing in geology, and it 

 connects itself with that idea of periods of creative activity 

 which we have already had occasion to notice. 



