EVOLUTION OF THE VERTEBRATES 159 



evolution of the teeth is important in mammalian evolution. 

 On the food habits of himself or his neighbors also depends an 

 animal's need for keen senses, defensive or aggressive structures, 

 and the development of powers of locomotion for escape from 

 enemies or for the capture of prey. Mammalian reproduction, 

 while it is distinctive, and to a limited degree varied within the 

 class, is by no means as important an indication of the course of 

 evolution as these other characters. 



Teeth. The fairly primitive teeth of man are an excellent illus- 

 tration of the possibilities of hcterodont dentition. The incisors 

 are sharp edged for cutting through tough tissues or biting off 

 pieces of food; they are effective in cutting flesh. The canines, on 

 the other hand, are of little or no use for anything but holding and 

 tearing. The broad molars are effective grinding structures by 

 which tissues of all kinds may be crushed and reduced to smaller 

 parts before swallowing. Carnivorous animals obviously have 

 greater need for cutting and tearing than for grinding teeth, hence 

 we find that even their molars are sharp edged shearing teeth. 

 Herbivorous species, on the other hand, have need for grinding 

 teeth in proportion to the harshness of their food; the grazing ani- 

 mals therefore have broad teeth with hard ridges for chewing 

 grains and grasses. Still other animals, like the anteaters, have no 

 need for teeth, which are correspondingly reduced or lacking. 



Structures and Habits. Correlated with food habits are other 

 structural characters. The carnivores have keen senses and speed 

 in order to detect and reach their prey, and powerful jaws and 

 sharp claws to aid in its capture. Herbivores find their teeth 

 poor defensive structures so their limbs are highly specialized for 

 speed without limitations imposed by other needs. If an ani- 

 mal finds food or protection in a different environment, its 

 limbs show the need of that environment, like the powerful fos- 

 sorial front legs of the mole, flippers of aquatic mammals, wings 

 of the l)ats, and arboreal adaptations such as the sharp claws of 

 the squirrels, hooked claws of the sloth, and prehensile append- 

 ages of the primates (Fig. 56). 



Classification. The classification of the Mammals is based 

 largely on such structural differentiation, although they are first 

 divided into the Prototheria, Metatheria and Eutheria partly on a 

 basis of reproductive functions. Members of the first group lay 

 eggs. The second are marsupials, including such animals as the 



