304 OUTLINES OF EVOLUTIONARY BIOLOSY 



typical pentadacfcyl limbs, is far more primitive than many other 

 mammalian groups. 



Each great group seems to have begun in a small way, then 

 developed rapidly, branching out in many directions and becoming 

 the dominant group for the time being, only to dwindle away 

 again and give place to some new and vigorous off-shoot. The 

 dominance of any particular group has often been accompanied 

 by the attainment of enormous size by its individual members, 1 



FIG. 150. Skeleton of Twoceratingena, from the Middle Eocene of Wyoming, 

 X ^V (From British Museum Guide, after O. C. Marsh.) 

 j 



and it is not impossible that this may h&ve had something to do 

 with its subsequent decline or complete extinction. 



So far as the animal kingdom is concerned, we may perhaps 

 say without exaggeration that the succession in time of the 

 different groups, as indicated by the geological record, amounts 

 "NJ to positive demonstration of the truth of the theory of organic 

 evolution. In the case of plants the record is perhaps not quite 

 clear, but here also there can be no reasonable doubt that the 

 /great groups succeeded one another in a manner consistent with 

 ^eo^, commencing with the algae and ending with the 

 flowering plants of the present day. 



1 A possible explanation of this fact is suggested in Chapter XXVI. 



