GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 315 



looking to members of the same class, naturalists are not unani- 

 mous which forms ought to be ranked as highest: thus, some look 

 at the selaceans or sharks, from their approach in some important 

 points of structure to reptiles, as the highest fish; others look at 

 the teleosteans as the highest. The ganoids stand intermediate 

 between the selaceans and teleosteans; the latter at the present 

 day are largely preponderant in number; but formerly selaceans 

 and ganoids alone existed; and in this case, according to the 

 standard of highness chosen, so will it be said that fishes have 

 advanced or retrograded in organization. To attempt to compare 

 members of distinct types in the scale of highness seems hopeless; 

 who will decide whether a cuttle-fish be higher than a bee — that 

 insect which the great Von Baer believed to be "in fact more 

 highly organized than a fish, although upon another type"? In 

 the complex struggle for life it is quite credible that crustaceans, 

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

 est mollusks; 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. Be- 

 sides these inherent difficulties in deciding which forms are the 

 most advanced in organization, we ought not solely to compare 

 the highest members of a class at any two periods — though un- 

 doubtedly 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 two periods. At an ancient epoch the highest and 

 lowest moUuscoidal animals, namely, cephalopods and brachio- 

 pods, 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 brachiopods, 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 number 

 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 complex relations, the standard 



