STATE OF DEVELOPMENT COMPARED 387 



to compare all the members, high and low, at the two periods. 

 At an ancient epoch the highest and lowest molluscoidal ani- 

 mals, namely, cephalopods and brachiopods, swarmed in 

 numbers; at the present time both groups are greatly re- 

 duced, 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 oppo- 

 site side, by considering the vast reduction of the brachio- 

 pods, and the fact that our existing cephalopods, though few 

 in number, are more highly organised than their ancient rep- 

 resentatives. 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 increase 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 fairness under such extremely complex relations, the 

 standard 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 extraordi- 

 nary 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 nat- 

 uralised there, and would exterminate many of the natives. 

 On the other hand, from the fact that hardly a single inhabi- 

 tant of the southern hemisphere has become wild in any part 

 of Europe, we may well doubt whether, if all the productions 

 of New Zealand were set free in Great Britain, any consid- 

 erable number would be enabled to seize on places now occu- 

 pied by our native plants and animals. Under this point of 

 view, the productions of Great Britain stand much higher in 

 the scale than those of New Zealand. Yet the most skilful 



