MAMMALIA 65 



are of great beauty, as, for example, in tigers and leopards. 

 As a general rule, the under side of the body is paler than the 

 rest (see p. 22). In the inhabitants of cold climates it frequently 

 happens that there is a well-marked distinction between summer 

 and winter coats in the matter of colour. All these different 

 arrangements are not haphazard, but bear a definite relation to 

 the habits of the animal, commonly rendering them inconspicuous 

 to foes or prey, &c. Such uses will be fully dealt with elsewhere, 

 when the colours of animals in general will be considered. 



(2) Milk. — One of the most noteworthy features regarding 

 Mammals is the care taken by them of their young, a character 

 which has much to do with the dominant position in the struggle 

 for existence which has been gained by this group of animals. 

 The word "gained" is used advisedly, for geological study shows 

 us that this leading place was in earlier periods occupied by types 

 of very different kind; e.g. the Reptiles constituted what may be 

 called the last reigning Vertebrate dynasty prior to the Age of 

 Mammals in which we now live. Just as in that age Man, the 

 newest comer, has, in virtue of his intellectual qualities, dethroned 

 his lower relatives and taken first place. 



It is a matter of common knowledge that in the early part of 

 their existence the young of Mammals are nourished by milk, a 

 substance which also plays, in various forms, a most important 

 part in the economy of human adults. As might be expected, 

 milk is an exceedingly nutritious substance, and may be taken 

 as the type of a perfect food. The composition of cow's milk is 

 approximately as follows: — 



Water, 87-0 



Solids, consisting of — 



1. Albuminoids (chiefly casein), 



2. Carbohydrates (milk-sugar), 



3- Fats, 



4. Inorganic salts, ... 



It will be seen (see p. 33) that all the necessary constituents of 

 animal food are present, and it may be added that they are in the 

 most advantageous proportions. 



A Mammal possesses one or more pairs of milk-glands, and 

 their position varies considerably in different species, but in all 



VOL. I. 



