INTRODUCTION. XV 



hoofed or ending in two blunt horny toes, as in Ruminants and 

 some Pachyderms; or blunt and almost entirely enclosed, as in the 

 Elephant and Camel. 



The temperature of Mammals being lower than that of Birds, 

 less effectual means are required of preserving the internal heat, 

 and accordingly, we find them provided with a covering more or less 

 dense of hair, or fur. In Monkeys and Bats there is only one kind 

 of hair, in most other animals there are both hair and wool, and 

 these vary in amount in different animals. Wool differs from hair 

 in having a serrated edge, as seen under the microscope, and on this 

 depends the quality of felting. It is generally more abundant in 

 animals living in cold countries, and is highly developed in all 

 the Himalayan Mammals. Hairs are of two kinds as regards their 

 growth. One kind grows continually and is never shed, as the mane 

 of the Horse. The second kind, of which is the fur of most animals, 

 grows to its full extent, and is shed and renewed periodically. This 

 causes a very considerable change in the general hue of many animals, 

 as well as in the amount of fur, so that the winter and summer 

 vestures are exceedingly dissimilar. 



A few animals have some of the hairs thick and strong (bristles), 

 and others have them flat and somewhat rigid, as in certain Rats. 

 Spines are found on Hedgehogs and Porcupines ; and a very few 

 Mammals are clad with scales or horny plates, as the Scaly Ant- 

 eaters and Armadillos. These spines and scales are all made up 

 of agglutinated hairs. Many Ruminants are adorned with horny 

 appendages on their heads, some of which are of the nature of 

 a horny sheath covering an internal bony cone ; others have no 

 internal nucleus and are renewed yearly. The former kind of horn 

 is, like the spines mentioned above, formed of agglutinated hairs ; 

 the latter partakes more of the nature of bone. 



In many animals the hairs are not uniformly coloured, but are 

 coloured differently at the base and the tip, and in some tribes are 

 ringed with different colours. The wearing down of these hairs 

 causes a difference in the external hue of such animals. 



In the Cetacea, which have no hair, the warmth of the body is 

 retained by a thick coating of fat or blubber. 



