Chap. XL] ANCIENT AND LIVING FOKMS. 119 



pods, swarmed in numbers ; at trie present time both 

 groups are greatly reduced, whilst others, intermediate 

 in organisation, have largely increased; consequently 

 some naturalists maintain that molluscs were formerly 

 more highly developed than at present ; but a stronger 

 case can be made out on the opposite side, by consider- 

 ing the vast reduction of brachiopods, and the fact 

 that our existing cephalopods, though few in number, 

 are more highly organised than their ancient represen- 

 tatives. We ought also to compare the relative propor- 

 tional numbers at any two periods of the high and low 

 classes throughout the world : if, for instance, at the 

 present day fifty thousand kinds of vertebrate animals 

 exist, and if we knew that at some former period only 

 ten thousand kinds existed, we ought to look at this in- 

 crease in number in the highest class, which implies a 

 great displacement of lower forms, as a decided advance 

 in the organisation of the world. We thus see how 

 hopelessly difficult it is to compare with perfect fair- 

 ness under such extremely complex relations, the stan- 

 dard of organisation of the imperfectly-known faunas of 

 successive periods. 



We shall appreciate this difficulty more clearly, by 

 looking to certain existing faunas and floras. From the 

 extraordinary manner in which European productions 

 have recently spread over New Zealand, and have seized 

 on places which must have been previously occupied by 

 the indigenes, we must believe, that if all the animals 

 and plants of Great Britain were set free in New 

 Zealand, a multitude of British forms would in the 

 course of time become thoroughly naturalised there, 

 and would exterminate many of the natives. On the 

 other hand, from the fact that hardly a single inhabitant 



