1885.] NATURAL SCIENCES OF PHILADELPHIA. 99 



earlier free-moving forms, which have been driven to wear heavy 

 armor for protection from stronger foes, and have been forced by 

 the weight and the character of this armor to take up a life on the 

 ocean bottom, either as stationary, crawling, or sluggishly swim- 

 ming forms. 



Where are the foes who have forced these forms of life into 

 degenerated conditions ? They are indicated in the rocks by no 

 hard parts, either offensive or defensive. They probabl}' needed 

 no protective armor, they had no internal hard skeletons, and the 

 only trace of early offensive weapons are found in the dubious 

 Conodonts, of the lower Silurian strata. Not until undoubted 

 fish teeth appear do we find unquestionable weapons of offense. 

 And there is no indication of active predatory swimmers until we 

 find the earliest fish remains. We may conceive that fishes had 

 so increased as to sweep the seas of any overabundance of food 

 forms, and had begun to actively prey upon each other. Then 

 they developed the protective armor to which they had previously 

 driven their prey. And this armor increased in thickness and 

 strength until the remarkable bony plates of the Devonian fishes 

 were produced. But in all probability several successive types 

 of life obtained mastership of the ocean, each superior form 

 driving all earlier forms to seek protection. Of these the fish 

 was the last and most powerful, and it cleared the open seas of 

 all competitors. 



Only from some such cause as this can we understand the sud- 

 den appearance of the Cambrian Orthoceratites, with their 

 bulky and clumsy shells, which certainty would never have been 

 developed except through pressure of sheer necessity. This 

 armor must have greatly diminished the motor powers of the 

 cephalopod ; it was solely protective in character, and it is 

 impossible to impute it to any cause save that of defense from a 

 powerful predacious foe. All the early lords of the ocean had 

 successively to clothe themselves in strong armor, or to vanish 

 from existence as more powerful forms appeared. 



There are strong indications, therefore, that in addition to the 

 armored forms preserved in the rocks, there was abundance of 

 naked forms of life, mainly swimmers, and pursuing a predatory 

 mode of life. If we pass backward through the succession of 

 fossil forms, it is to find the armored types decreasing in numbers 

 and variety. We seem to gradually approach a period in which 

 the naked swimming forms were greatly in excess. This may 

 have been preceded by a period in which there were no armored 

 forms. In such a case, though life may have been as abundant as 

 now, it could not have been fossilized. Such ma} r possibly have 

 been the pre-Cambrian life condition. 



There could have been no era of life, indeed, in which preda- 

 tory forms did not exist. But there may have been a long period 

 during which animals were incapable of secreting armor. The 



