386 PROCEEDINGS OF THE ACADEMY OF [1885. 



animal life is no longer to assume armor, but to throw off armor, 

 and return towards the unprotected condition. This tendencj' 

 was quite as marked in its operation as the others, as a hasty 

 review will show. 



In the antique t}-pe of vertebrate life, the fish, the thick armor 

 of the primeval era has been in great part replaced b} r the thin 

 scales of the Teleosteans. The Ganoids have nearly vanished. 

 Man}' Elasmobranchs yet exist, but their armor never gained the 

 dense and rigid character of the Ganoid scales. But the loss of 

 the old condition is more particularly shown in the new forms of 

 life. The Labyrinthodont amphibians were clothed in armor, 

 their heads in particular being protected by hard bony plates. 

 Modern amphibians are naked-skinned animals. The reptiles are 

 usually scaled, but, with the exception of the crocodiles and 

 turtles, and some few fossil types, do not seem to have ever been 

 clothed in bony armor. In the later vertebrate classes, the birds 

 and mammals, all defensive armor is lost, the covering of hairs 

 and feathers being protective only against cold. Finally, in the 

 human species, even the covering of hairs is nearly lost, and in 

 external condition the highest form of animal life approaches the 

 lowest. 



A like tendency to pass from the armored to the unarmored 

 condition appears in invertebrate life. In most of the inverte- 

 brates the dermal covering serves as a basis of muscular attach- 

 ment, and cannot be dispensed with. The soft-bodied invertebrates 

 of low orders, such as the worms, the medusae, etc., are probably 

 survivals of the primitive life condition, and may indicate the 

 general character of pre-Cambrian life. But in the higher mol- 

 lusks a very interesting variation appears. The Palaeozoic 

 cephalopods were all covered with a dense protective armor. In 

 the Mezozoic period this class began to give way to an unarmored 

 class, with a change in the character of its muscular attachment. 

 To the tetrabranchiates, with muscles attached to the external 

 shell, were added the dibranchiates, with naked surface, and an 

 internal basis of muscular attachment. Since that period the 

 evolutionary process has been highly interesting. The armored 

 cephalopods have gradually disappeared, until only the Nautilus 

 remains. The unarmored forms have rapidly increased, until they 

 abundantly people the modern seas. 



The process of modification I have here briefly indicated has 



