ANCIENT AND LIVING FORMS. 343 



fche most important element in striking a balance — but we 

 ought to compare all the members, high and low, at two 

 periods. At an ancient epoch the highest and lowest mol- 

 luscoidal animals, namely, cephalopods and brachiopods, 

 swarmed in numbers; at the present time both groups are 

 greatly reduced, while others, intermediate in organization, 

 have largely increased ; consequently some naturalists main- 

 tain that mollusks were formerly more highly developed 

 than at present ; but a stronger case can be made out on the 

 opposite side, by considering the vast reduction of brachio- 

 pods, and the fact that our existing cephalopods, though 

 few in number, are more highly organized than their ancient 

 representatives. We ought also to compare the relative 

 proportional 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 thou- 

 sand kinds existed, we ought to look at this increase in num- 

 ber in the highest class, which implies a great displacement 

 of lower forms, as a decided advance in the organization of 

 the world. We thus see how hopelessly difficult it is to 

 compare with perfect fairness", under such extremely com- 

 plex relations, the standard of organization of the imper- 

 fectly known faunas of successive periods. 



We shall appreciate this difficulty more clearly by look- 

 ing to certain existing faunas and floras. From the ex- 

 traordinary 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- 

 uralized there, and would exterminate many of the natives. 

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

 itant of the southern hemisphere has become wild in any 

 part of Europe, we may well doubt whether, if all the pro- 

 ductions of New Zealand were set free in Great Britain, any 

 considerable number would be enabled to seize on places 

 now occupied 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 naturalists, from an examination of the 

 species of the two countries, could not have foreseen tkia 

 result, 



