CHAPTER VI 



THE MAMMALS AND MAN 



THE subject of our discussion is now narrowed down to the group 

 of the mammals. The mammals are characterised by two very 

 obvious features : a body-covering of hair, and a set of special 

 glands in the female which secrete milk for the nourishment of 

 the young. These are constant characters, and neither is ever 

 found in any other group. As to how the hair originated, nothing 

 definite is known ; but (while there are certain difficulties in 

 regard to the theory) it is on the whole reasonable to suppose that 

 the mammalian hair arose, as the bird's feather undoubtedly 

 did, as a modification of the reptile's scale. The mammary 

 gland appears to represent a modification of other skin glands, 

 either of sweat glands or more probably of the oil glands which 

 exist in connection with the hairs. 



Another important character, already mentioned at the end 

 of the last chapter, is the diaphragm, a muscular partition separat- 

 ing the body cavity into a thoracic and an abdominal portion. 

 The diaphragm has important functions in connection with the 

 mechanism of breathing. By means of it the thoracic cavity 

 can be increased or diminished in size, and air thus drawn into 

 or expelled from the lungs. It is interesting to observe that a 

 similar partition, with the same function, occurs in the Croco- 

 diles, but this has different relations to the abdominal organs, 

 and has evidently evolved quite independently. 



Very characteristic of the mammals are, further, their teeth, for 

 whereas the teeth of the reptiles are indefinite in number, and 

 generally very numerous, those of the mammal are relatively 

 few, and each species has a definite normal number. Moreover, 



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