Seite - 387 - in The Origin of Species

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