1885.] NATURAL SCIENCES OP PHILADELPHIA. 387 



another interesting feature, which may be pointed out. This is 

 that the modification has not taken place by a simple change in 

 the dermal structure of existing types, but that this change has 

 been accompanied by a radical change in organic structure. The 

 representatives of the old forms have retained much of their old 

 surface structure. The radical variation in surface condition has 

 been confined to new types of life. 



In the case of the primitive soft-bodied animals, for instance, 

 they have probably had representative forms throughout the 

 whole era of life, and may be closely simulated by the soft-bodied 

 ocean animals } r et existing. The assumption of armor by certain 

 forms was probably accompanied by a marked change in structure, 

 the dermal variation being co-related with other important 

 changes. Of this, of course, we cannot be sure, but in the parallel 

 case of the discarding of armor this idea holds good. The 

 Ganoids continue armored fishes to the present day. The pre- 

 vailing thin-scaled fishes are of a new structural type. In the 

 cephalopotls we do not find a simple discarding of shells by the 

 armored type, but the gradual disappearance of this type, and its 

 replacement by a type of markedly different structure. In the 

 vertebrata generally the antique types have preserved the scaled 

 condition to a greater or less extent. It is in the new structural 

 types, the birds and mammals, that this antique condition has been 

 most fully discarded, and replaced by a radically distinct dermal 

 covering. It would almost seem as if it had been impossible for 

 any type of animal to completely dispense with a primitive struc- 

 tural feature except under the influence of a general organic 

 change. In the assumption of armor the whole organic structure 

 may have suffered a correlated change. In the discarding of 

 armor a like radical change in structure has taken place, while 

 the representatives of the ancient types have preserved their 

 ancient dermal conditions. 



I present these simply as a series of well-known facts. It is 

 with the cause of these facts that I am mainly concerned. Why 

 did animal life exist for a long period without protective covering ; 

 then adopt armor of defense, and develop it to an extraordinary 

 degree ; and finally slowly discard this armor, and return towards 

 the unprotected condition ? We have here a remarkable series of 

 evolutionary changes. They undoubtedly had sufficient and 

 powerful causes. What were those causes ? 



