The mammals with few exception (like cetaceans, anteaters, etc.) are characterized by hetrodont dentition. There are four types of teeth; the front ones are incisors being prehensile, the canines for defense and offense, premolars and molars or grinding teeth. The premolars and molars show the greatest structural modifications to meet the requirements of possessor.
The insectivorous type, those of the stem form, are low crowned, simple with few cusps, generally sharp-pointed and suitable for crushing feeble prey. In the carnivorus, the cheek-teeth become high-crowred, trenchant, shearing structures. Carrion-feedings forms have as a rule, blunter teeth. In fish-eating creatures like the seals the teeth have become secondarily simplified and all prehensile.
The toothed whales (Odonotceti) have lost their tooth differentiation, thus all the teeth are alike, simple, slightly recurved, grasping cones. In the sperm whales (Physeter) the teeth are absent in the upper jaw and in the whalebone whales (Mystaceti), the teeth are absent from both the jaws, except for functionless vestiges in the unborn young, and their place in taken by the curious baleen or whalebone.
Herbivorous types have the incisors fitted for seizing and cutting tion The canines are of little importance. The premolars and s are modified in different forms. They may be short crowned (jgrachyodont) for succlient leaves of twigs or long-crowned (hypsodont) for harst grasses.
The myrmecophagus types represent the height of specialization, for in its extreme development of teeth have utterly disappeared, the jaws are reduced, the mouth is incapable of opening except at the extreme anterior end and has become tubular, with highly extensile and prehensile tongue. The tongue is provided with sticky substance. It is thrust into an ant-hill and withdrawn which brings a great number of insects which are swallowed without mastication.
Thus we find that the living beings have the wonderful plasticity, which, in their efforts to find food and safety, become adapted in the course of time to all possible conditions of life i.e. earth, air and water. These adaptations are chiefly concerned with those parts most closely in contact with the environment-the feet for safety and teeth for food.
The soft internal parts are more conservative and do not change; except those are again directly concerned with food or locomotion. We find that nature does repeat herself, but never exactly, nevertheless some remarkable convergences of unrelated animals also recorded.
It has been established that a highly specialized or adapted form became stereotyped and incapable of radical changes, and with altering condition they could not adjust themselves and became extinct, while a less specialized and therefore more plastic race, meeting the changed requirements, survives.