290 GEOLOGY. 



protection was needed. Can the known Cambrian animals have con- 

 stituted such enemies, or must others be supposed to have existed 

 to furnish a good reason for such protective provisions? Perhaps the 

 most significant feature of the protective devices lies in the fact that 

 they are usually of the same type as those possessed by the corre- 

 sponding animals of later times. The shells of the gastropods, pelecy- 

 pods, and brachiopods differ from those of to-day only in minor fea- 

 tures. The coverings of the trilobites are very much the same as 

 those of their crustacean relatives in existing seas. Much the same 

 may be said of nearly every form. If there had been a radical change 

 in the character of their enemies or rivals, we might expect some nota- 

 ble change in the defensive devices. It is a natural inference, there- 

 fore, that the conflicts of life in the Cambrian waters had become very 

 much what they have been in later times. The inference may be 

 pushed a step further, and the deduction drawn that the conflicts 

 which led to the evolution of the defensive devices were much the 

 same that they have been throughout the period of their retention. 

 If the reign of the great predaceous mollusks, the cephalopods, and 

 that of the predaceous fishes be supposed to have reached back through 

 the Cambrian period to the time when the armorings of the various 

 forms were acquired, the total assemblage of attackers and defenders 

 in the seas would have been very much the same as it is now, for the 

 marine reptiles and mammals that have come in since have not essen- 

 tially changed the nature of the conflict. The occurrence of the ceph- 

 alopods in the closing stage of the Cambrian, and of the fishes in the 

 succeeding Ordovician period, removes any special improbability 

 from the hypothesis that they were present in the early Cambrian, 

 and perhaps even earlier. 



If the defensive devices of the Cambrian animals are to be explained 

 within the strict limits of the known forms, the armor of the large 

 trilobites must apparently be regarded as a defense against mem- 

 bers of their own race, for no other known animal apparently had 

 sufficient celerity of motion and sufficient strength to give occasion 

 for such protection. A similar remark may be made of the great 

 cephalopods when they came in toward the close of the period. The 

 protection of the gastropods, pelecypods, brachiopods, etc., of the 

 early Cambrian may have been needed as a defense against the trilo- 

 bites, but it is not at all clear that just such coverings as they had 



