1885.] NATURAL 8CIENCE8 OF PHILADELPHIA. 389 



selection. It would give them an advantage in the struggle for 

 existence, which could not be overcome until the carnivora ha<l 

 gained a correlated development. On the other hand any new 

 offensive weapon or method would give the carnivora an advan- 

 tage, which would render necessary some new defensive adaptation 

 in food animals. 



In this process of evolution we find several instances in which 

 defensive appliances seem to have gained a special development, 

 which were only slowly met by new methods of attack. Such I 

 conceive to have been the case when the power of secretion of 

 dermal armor was once attained, and to the high value of this 

 defensive expedient I ascribe its rapid development. According 

 to m} r theory of the case the preceding animals had been naked 

 skinned, and the destructive weapons of the carnivora such as 

 were adapted to the capture of soft-bodied pre}'. There is not a 

 shred of evidence that any toothed forms existed preceding or 

 during the Cambrian era, nor until well on in the Silurian. Yet 

 toothless animals could not easily overcome animals with a strong 

 covering of bone or other hard material. It is to this fact that I 

 ascribe the rapid increase in number and variety of armored 

 forms. Their armor gave them a special advantage in defense, 

 and under this idea there is no difficulty in understanding the 

 very rapid and general evolution of this structural feature of 

 animal life. 



In fact, as a result of the development of defensive armor, a 

 discrepancy arose between the agencies of attack and defense. 

 Defense had the best of it. A structural feature had been rapidly 

 acquired, which could only be met by some corresponding new 

 means of attack, and this new carnivorous weapon took a long 

 time to develop. 



The new weapon, through whose aid the aggressive again 

 slowly matched thedefensive appliances, made its appearance in 

 the form of the tooth. It but slowly grew effective. The first 

 evidence we find of teeth are the minute conodonts, which may 

 or may not have had this function. Fish teeth at first appear in 

 small and weak forms, but they gradually grow large and powerful , 

 and well adapted for cutting and crushing. A race between 

 aggressive and defensive powers apparently took place. Armor 

 grew thicker and denser as teeth became more efficient. As one 

 result of the race we have the great Devonian fishes, with their 



