'Chap. xi. A ncient and Living Forms. 309 



own class, might beat cephalopods, the highest molluscs ; and such 

 crustaceans, though not highly developed, would stand very high in 

 the scale of invertebrate animals, if judged by the most decisive of 

 all trials— the law of battle. Besides these inherent difficulties in de- 

 ciding which forms are the most advanced in organisation, we ought 

 not solely to compare the highest members of a class at any two 

 periods— though undoubtedly this is one and perhaps the most 

 important element in striking a balance— but we ought to compare 

 ^11 the members, high and low, at the two periods. At an ancient 

 epoch the highest and lowest molluscoidal animals, namely, cephalo- 

 pods and brachiopods, swarmed in numbers ; at the 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 con- 

 sidering the vast reduction of brachiopods, and the fact that our 

 existing cephalopods, though few in number, are more highly orga- 

 nised 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 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 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 multi- 

 tude 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 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 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 



