388 PROCEEDINGS OF THE ACADEMY OP [1885. 



These we need not go far to seek. They are plainly evident. 

 The variations described have taken place under the influence of 

 one of the most active and efficient agents in evolution, that of 

 the reciprocal influence of attack and defense on animal structure. 

 To this agent in animal change there has not been given so much 

 attention as it demands. It is, indeed, but one out of numerous 

 agents which act to modify animal form and structure. But 

 among these agents it has been one of the most active, efficient 

 and unceasing. 



The effort of food animals to escape from carnivorous foes, has 

 given rise to a great variety of defensive tissues, functions, and 

 habits. It has aided in the natural selection of armor of various 

 kinds, of defensive weapons, of speed and flexibility of motion, 

 of size and strength of body, of cunning and alertness, and of 

 many other characteristics, each of evolutionary importance. 

 And the study of animal development goes to show that these 

 varied conditions have been assumed successive!} 7 . Nature has 

 apparently fully worked out the capabilities of one mode of 

 defense before proceeding to another. 



The effort to capture and destroy food animals has been equally 

 important as an agent in evolution. It has caused the natural 

 selection of weapons of offense, such as claws and teeth, of 

 strength, swiftness, agility, alertness, cunning, and the like attri- 

 butes of mind and body, together with many special habits, all of 

 which form steps in evolution. The main cause of this advance 

 has been the reciprocal action of these agencies. If a food animal 

 gained some structural feature which gave it an advantage over 

 its carnivorous foes, the latter would be at a disadvantage until 

 they had gained equivalent features. So if a carnivorous animal 

 gained some habit, motion or weapon which gave it an advantage 

 in destroying, this must have acted as an incitement to a corre- 

 sponding development in food animals. Natural selection has, in 

 both cases, preserved the forms best adapted to the new condi- 

 tions of attack or defense, and the carnivora and herbivora have, 

 in a metaphorical sense, pursued each other up the ladder of 

 evolution. 



In this process now one class, now the other, may have taken 

 the lead. If at any time the two classes were evenly balanced in 

 powers of attack and defense, any new power of defense or escape 

 in the food animals would undoubtedly be preserved by natural 



