HERBIVOROUS DINOSAURS 225 



step by step, with a great bony frill protecting the neck. This 

 evolution took place stage by stage with the evolution of the 

 predatory mechanism of the carnivorous dinosaurs, so that 

 the climax of ceratopsian defense {Triceratops) was reached 

 simultaneously with the climax of Tyrannosaurus offense. This 

 is an example of the counteracting evolution of offensive and 

 defensive adaptations, analogous to that which we observe 

 to-day in the evolution of the lions, tigers, and leopards, which 

 counteracts with that of the horned cattle and antelopes of 

 Africa, and again in the evolution of the wolves simultaneously 

 with the horned bison and deer in the northern hemisphere. 

 It is a case where the struggle for existence is very severe at 

 every stage of development and where advantageous or dis- 

 advantageous chromatin predispositions in evolution come con- 

 stantly under the operation of the law of selection. Thus in the 

 balance between the reptilian carnivora and herbivora we find 

 a complete protophase of the more recent balance between the 

 mammalian carnivora and herbivora. 



The climax of defense was reached, however, in another 

 line of Predentata, in the herbivorous dinosaurs, known as 

 Ankylosaurus, in which there developed a close imitation of the 

 armadillo or glyptodon type of mammal, with the head and 

 entire body sheathed in a very dense, bony armature. In 

 these animals not only is motion abandoned as a means of 

 escape, but the teeth become diminutive and feeble, as in most 

 other heavily armored forms of reptiles and mammals. The 

 herbivorous function of the teeth is replaced by the develop- 

 ment of horny beaks. Thus these animals reach a ground- 

 dwelling, slow-moving, heavily armored existence. 



