1885.] NATURAL SCIENCES OP PHILADELPHIA. 385 



ATTACK AND DEFENSE AS AGENTS IN ANIMAL EVOLUTION. 

 BY CHARLES MORRI8. 



In considering the development of the dermal skeleton of 

 animals, with its various modifications, we are led almost to the 

 conception that nature has been controlled at successive periods 

 by special ideas, each dominant during a long period, and then 

 abandoned in favor of a new one. I have, in a previous commu- 

 nication to the Academy, advanced the hypothesis that in the 

 primitive life era animals were destitute of hard parts, either 

 external or internal, and that to this we must ascribe the lack of 

 primitive fossils. 



The development of an external skeleton, which seems to have 

 long preceded that of an internal one, came like a new idea to 

 nature, which was adopted almost simultaneously as it seems, 

 though probably at considerable intervals, by the various t}*pes 

 of life. We are quite sure that the first appearance of fossils in 

 the rocks does not indicate the first appearance of life upon the 

 earth. Early fossilization is due to the preservation of the 

 dermal skeletons of animals of considerably advanced organiza- 

 tion, and these were very probably preceded, during a long era, 

 by soft-bodied forms of low organization. These could leave no 

 trace of their existence, except in the case of the burrowing 

 worms, or of impressions made by animal forms on beds of mud 

 or other plastic material. Yet after the advent of armored ani- 

 mals, it is probable that the seas were still tenanted by numerous 

 soft-bodied forms, mainly swimmers, the progenitors of the many 

 naked ocean swimmers which still exist. 



The earliest armored forms were principally surface dwellers, 

 or sluggish swimmers. Swift-swimming armored animals came 

 in with the fishes, and these increased in thickness and weight of 

 armor to the end of the Devonian era. During this period all the 

 higher forms of life seem to have acquired more or less dense 

 dermal armor. Their agility must have been much reduced by 

 the weight and rigidity of this armor. None but the fishes were 

 active swimmers, and most of the armored animals were surface- 

 dwellers. 



If now we come down to a later era of life, we find in operation 

 what seems a third idea of nature. The prevailing tendency in 

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