25 



MAMMALS 



\Vk have studied the rabbit as an example of a mammal, and 

 in this chapter we shall consider the class Mammalia as a whole. 

 In spite of the relatively small numbers of species (Britain has 

 about fifty, while there are nearly two hundred resident birds and 

 some twenty thousand insects) its members have been extra- 

 ordinarily successful in colonising almost all parts of the globe, 

 largely because their constant temperature makes them to a great 

 extent independent of the weather, and because their brain makes 

 them able to learn by their experience and modify their behaviour 

 in an unprecedented way. The mammals of different environ- 

 ments have different details of structure which on the whole are 

 obviously connected with habit or habitat ; thus otters, which 

 swim, have webbed feet, and many monke3^s, which climb, have 

 prehensile tails. The existence of such ' adaptations ' is now 

 considered evidence that all the mammals have evolved from the 

 same ancestors (Chap. 30), and such divergence of descendants 

 is known as adaptive radiation. Most groups of animals show such 

 radiation, and it is a useful intellectual exercise for the student, 

 having studied it in mammals, to try to apply the principle to 

 other animals which he knows. The more successful a group is, the 

 more likely is it to show clear adaptive radiation ; the insects and 

 birds are therefore easy to work out. It should be noted that 

 adaptive radiation is best seen in external features, since these 

 have a closer dependence on the external environment than has 

 the internal structure. 



In formal terms the mammals are not easy to separate from 

 reptiles, but for practical purposes an animal is easily recognised 

 as a mammal if it has hair, or, should only the skeleton be avail- 

 able, if it has a lower jaw consisting of a single bone on each side, 

 or in dissection if it has a muscular diaphragm. The majority 

 of existing mammals have also two other features which are 

 possessed by no other animals — a placenta formed from the 

 allantois, and mammary glands, which are vestigial in the male 

 but in the female produce milk on which the young are fed. 

 Mammals which possess both these features are called Eutheria 

 or Placentaha. The British Isles contain examples of almost all 



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