INTRODUCTORY. 7 



always more or less clothed with hair. The heart has a 

 single left aortic arch, the blood is hot, and the heart and lungs 

 are lodged in a special cavity separated from the abdomen by 

 a muscular partition known as the diaphragm. 



Respecting one item in the foregoing it has been said truly 

 that the possession of a few or many true hairs as outgrowths 

 from pits in the skin is alone sufficient to distinguish a Mammal 

 from any other animal. Although these hairs may take different 

 forms, they are alike in their origin even, to take an extreme 

 case, the spines of the Hedgehog. Each hair consists of an 

 outer wall enclosing a central cavity filled with pith, in which is 

 the dark pigment which gives the hair its colour. In the 

 Mammals this pigment is always brown, and the varying tints 

 of the hairs black, brown, tawny, cream-colour or white 

 depends upon the amount of pigment and its disposition in the 

 pith, combined with differences in the density of the envelope. 

 In some cases, as about the mouth, eyes, and ears of the Cat ? 

 long sensitive hairs are connected with the terminations of 

 nerves, which help the animal to feel its way. There are no 

 marked colour differences in the fur of the sexes, such as we 

 find in the plumage of Birds ; though we do find such dis- 

 crepancies in the presence or absence of horns in Deer, and in 

 the manes and hair-tufts of some exotic Mammals. Certain 

 species, such as the Alpine Hare and the Stoat, undergo a 

 marked seasonal change of colour in the fur under the influence 

 of low temperature. This may be quite sudden, owing to a 

 rapid fall of temperature, and as shown by Metchnikoff is 

 effected by the pigment granules being consumed by a sort of 

 phagocyte. By Metchnikoff's researches an old controversy 

 appears to have been settled finally. 



