142 THE ASCENT OF MAN 



Other way of bridging a space than these three, the 

 Lintel, the Round Arch, the Gable ; they may vary 

 the curve of the arch, or curve the sides of the 

 gable, or break them down ; but in doing this they 

 are merely modifying or sub-dividing, not adding 

 to the generic form."^ 



In some such way, there may be terminal 

 generic forms in the architecture of animals ; and 

 the persistent types just named may represent 

 in their several directions the natural limits of 

 possible modification. No further modification of a 

 radical kind, that is to say, could in these instances 

 be introduced without detriment to practical effici- 

 ency. These terminal forms thus mark a normal 

 maturity, a goal ; they represent the ends of the 

 twigs of the tree of life. 



Now consider the significance of that fact. 

 Nature is not an interminable succession. It is 

 not always a becoming. Sometimes things arrive. 

 The Lamp-shells have arrived, they are part of the 

 permanent furniture of the world ; along that par- 

 ticular line, there will probably never be anything 

 higher. The Star-fishes also have arrived, and the 

 Sea-urchins, and the Nautilus, and the Bony 

 Fishes, the Tapirs, and possibly the Horse — all 

 these are highly divergent forms which have run 

 ^Stones of Venice^ ii. 236. 



