126 THE ORIGIN OF SPECIES 



plex struggle for life it is quite credible that crustaceans, 

 not very high in their own class, might beat cephalopods, 

 the highest niollusks; 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 diffi- 

 culties in deciding which forms arc the most advanced in 

 organization, we ought not solely to compare the highest 

 members of a class at any two periods — though undoubt- 

 edly 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 molluskoidal ani- 

 mals, 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 maintain 

 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 brachi- 

 opods, and the fact that our existing cephalopods, though 

 few in number, are more highly organized than their an- 

 cient representatives. We ought also to compare the rela- 

 tive 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 ani- 

 mals exist, and if we knew that at come former period 

 only ten thousand kinds existed, we ought to look at 

 this increase in number in the highest class, which im- 

 plies 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 fair- 



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