Chap. X. ANCIENT AND LIVING FORMS. 315 



taceaus, not very high in tlrcir own class, might beat cephalo- 

 pods, tlic highest nioUusks ; and such crustaceans, though not 

 highlv devek)pccl, wouhl stand very high in the scale of inverte- 

 brate animals if judged by the most decisive of all trials — the 

 law of battle. Besides these inherent difficulties in deciding 

 which forms are the most advanced in organization, we ouglit 

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

 two periods — though undoul)tcdly this is one and perhaps the 

 most important element in striking a balance — but we ought 

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

 At an ancient epoch the highest and lowest mollusks, namely, 

 cephalopods an(l brachiopods, swarmed in numbers ; at the pres- 

 sent tim(^ both ordei*s are greatly reduced, while other orders, 

 intermediate in organization, have largely increased; conse- 

 quently some naturalists maintain that mollusks were formerly 

 more highly developed than at present ; but a stronger case 

 Clin be made out on the opposite side, by considering the vast 

 reduction of the lowest mollusks, and the fact that our existing 

 cephalopods, though few in nimibcr, arc more highh'' organized 

 than their ancient representatives. We ought also to compare 

 the relative proportional numbers of the high and low classes 

 throughout the world at any two periods : if, for instance, at 

 the present day fifty thousand kinds of A-ertebrate animals ex- 

 ist, 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 org-anization of the 

 world. AVe thus see how hopelessly dillicnlt it is to compare 

 with perfect fairness, under such extremely complex relations, 

 tlw; standard of organization of the imperfectly-known faunas 

 of successive j^eriods. 



We shall apjireciatc this difficnlly the more cleaily, by 

 looking to certain existing faimas and iioras. From the extraor- 

 dinary manner in which European productions have recently 

 spread over New Zealand, and have seized on places whicli 

 must have lieen previously occupied, we must believe that, if 

 nil the animals and plants of Great Britain were set free in 

 New Zeahuul, in the course of time a multitude of ]3ritish forms 

 would become thoroughly naturaliz("d there, and would exter- 

 minate many of the natives. On the other hand, from hardly a 

 single inhabitant of the southern hemisphere having become 

 Anld in any jiart of Emope, we may well doubt whether, if all 

 the productions of New Zealand vicrc set free in Great Britain, 



