FISHES OF THE CARBONIFEROUS SYSTEM. 81 
Very few living Sharks have dorsal spines, but their abundance in some of the geo- 
logical formations seems to indicate that they were worn by a majority of the ancient 
Sharks. The significance of this apparent difference we cannot fully comprehend at 
present, but it has doubtless an important zoological meaning. The changes in the 
“habits of good society,” as illustrated by our history during the last two centuries) 
may perhaps help us to explain the phenomenon. A hundred years ago every gen- 
tleman wore his small sword, and was so prompt and skillful in its use, that he who 
was without the weapon or the power to wield it was at a great disadvantage among 
his fellows. When, therefore, the habit was general, it was necessarily universal; but 
now a general disarmament has pnt all members of society on an equality. In review- 
ing the various phases of armament, offensive and defensive, we find such an infinite 
variety and so many changes of style, that it almost seems that the caprice of fashion 
ruled the world in tormer times as now; but all tiis diversity was doubtless controlled 
by profound physiological laws. There can be little question that both utility and 
beauty took part in producing the varied results; that sexual selection and the sur- 
vival of the fittest combined to produce the variety we see. A curious parallelism is 
discoverable in the changing styles and in the effectiveness of contemporaneous offen- 
sive and defensive armor, and we see that all through the ages the same contest has 
been maintained that is now going on between improved projectiles and plate armor. 
In the Middle Ages, for defense against the spear, sword, and mace, 
chain, scale, or plate armor was adopted, and the latter was finally made so 
nearly impenetrable, that it rendered the wearer almost invulnerable; but 
such a suit of armor was so heavy as to prevent all free and rapid move- 
ment, and indeed in some cases to render it difficult for an overthrown 
warrior to rise from the ground. The introduction of fire-arms rendered 
scale or plate armor practically useless, and it was speedily abandoned. 
This change also involved the adoption of a new system of tactics, and 
celerity of movement with the musket, bayonet, and artillery has become 
the important factor in modern warfare. So in the history of the Elasmo- 
branchs, we find that the negative defenses of spines and dermal tubercles 
. have been subordinated to the teeth, the chief organs of attack; the cutting 
and piercing succeeding the crushing teeth, Otodus, Carcharodon, Lamna, and 
Odontaspis superseding the Cestracionts, Psammodus, Orodus, Cochliodus, ete. 
The Placoderms, the ruling dynasty in the Devonian seas, though clad 
in armor that made them invulnerable, were heavily handicapped by the 
defenses which they carried about, and it is therefore not surprising that 
they were superseded by the swifter and more flexible Sharks and Teleosts. 
MON. XVI——6 
